Sanity check on my plans for a DIY darkroom sink?

I've spent a bunch of time reading threads here about making a darkroom sink, and I think I'm ready to spring for materials on mine. I've been making do with a discarded IKEA shelf screwed onto a metal aquarium stand, all sitting in an unused bathtub in my bathroom-turned-darkroom, for a few years now. More of a darkroom table than a sink.

The plan is, a 2'x4' piece of 1/2" marine plywood, with 5" walls made of the same material (glued and screwed into place). Then a good bead of silicone caulk along all the inside corners, and a few coats of waterproof porch paint to finish it off. Once cured, a drain can be added to one side,and the whole thing can be permanently affixed to a stand made of 2x4s. The stand will allow me to tilt the sink for drainage.

Will this caulk work for my corners? If not, make me a recommendation (preferably from Home Depot)?

Will this paint work for permanent waterproofing? If not, make me a recommendation (preferably from Home Depot)?

Very comfortable with the woodworking aspect of building this. Very open to advice from people who know more about the sealing/painting aspect than I do as far as the materials I'm using.

I used vinyl decking material to line my plywood sinks for my darkroom .... worked like a charm & beats breathing epoxy or buying a respirator

I think you need to paint first. Caulk and then paint will cause paint to peel off along the edges.

I wouldn't paint marine plywood. I would probably use non-treated.

So standard plywood, then waterproof paint (is this porch stuff okay?) and lastly caulking the inner joints?

The paint might work, I would plan to do as many coats as possible. For a joint sealer I would opt for 5200. It is expensive, but it will never come off and will never leak.

I have several suggestions for a homemade plywood sink. Mine has been in use for fifty years. Marine plywood, seal and reinforced seams with fiber glass, 10 inch deep to prevent splashes, make removable covers for more counter top when not used as a sink, faucets at the opposite end from the drain, cartridge water filter in the water line and paint it with marine polyester to which a color can be added. If possible a storage shelf underneath.

I don’t know why you wouldn’t paint marine plywood- it’s made to be painted and sealed (at least that’s my experience with wooden boats).
I think if I were doing this I would use an epoxy paint and use a sealer for the corners compatible with the paint. Years ago I made a sink much like the one you are proposing, 1/2” plywood with a base made from 2x4’s with a 1/2” slope per foot. No drain, but me end was open which I attached to a plastic freestanding laundry sink. I used fiberglass mat to line it. It lasted forever, even outside before I had to dump it because no more darkroom. My handyman made my current one, it’s made out of shower stall underlayment, thick plastic sheeting stapled to the sides made of 2x4’s. Not exactly elegant, but totally functional and no leaks because it’s one continuous piece. Shower drain on one end.
Good luck on your project, I’m sure whatever plan you go with it will work fine. The important thing is developing your film and prints!

As a moderator, I always like it when I see a thread asking for a "sanity check"!
But as a darkroom user, and continuing on with jeffreyg's excellent suggestions:
- a surface that isn't slick and slippery makes it easier to place things and have them not move on their own;
- consider incorporating easily removable plastic or wire shelving material that allows trays to be off the sink bottom;
- consider cushioned material on the adjacent floor;
- set the height carefully to match your height and where your arms rest comfortably;
- make sure the edges are smooth and comfortable to lean on;
- a bar rail to rest one foot on is nice; and
- allow for a ledge to support hoses that can be used to move water to various parts of the sink .

I built mine in a similar way, but used a cheaper grade of waterproof plywood, all coated with epoxy. 15 years old and no problems.

There are folks who will poo-poo the homemade plywood sink idea. I will not.
I will poo-poo the “porch and floor” paint though.
I didn’t think it will keep up with any chemicals.
I don’t have many better suggestions that aren’t high voc.
I’m also not sure which plywood HD stocks.
I wouldn’t necessarily use true “stamped” marine grade as it’s eye wateringly expensive. But at least a plywood made with good glue that has filled voids and one “a” face.
I too would go with epoxy I think.
Or see if a local Line-x outfit will coat your sink for you.

I would ask a professional paint dealer. Epoxy paint, the good stuff is pretty bulletproof. Sounds like you have a good understanding of the woodworking skills, that's 90% of the issue. Keep the bottom smooth so when you are finished you can squeegee the bottom dry.

First of all, real marine plywood is VERY expensive at the moment. Second, it ideally needs to be treated with a 2-part penetrating marine epoxy too, likewise expensive, and damn unhealthy to work with in an indoor environment. Before retirement, I was the buyer for a supply company which acquired and sold more of this kind of epoxy than all the rest of their clients combined. We sold huge volumes of
the nasty stuff to the Navy, private marine repair businesses, home restoration specialist, and even for sake of a considerable number of home as well as institutional darkrooms over the decades. Stacks and stacks of plywood were in the next building over, plus another entire warehouse.

But what did I personally do when it came to my own sink? I had mine heat-welded out 1/4 inch thick polypropylene sheeting instead. Cheaper, impervious to chemicals, light enough to carry inside the building, even at 10 ft long by over 3 ft wide, replete with ledge at the back.

If you do go the plywood route, even epoxy paint might not hold up well unless a penetrating liquid epoxy is used first. It's inevitable that I'll get a bit of flack for saying that based on the anecdotal personal experiences of a few. But I would not be exaggerating at all by countering that my own perspective takes in 10,000 times as much surface area with respect to long-term performance issues. I've even sold the Navy and Defense contractors in the Pacific types of coatings of such high performance combined with health hazard that they're illegal to apply unless total outside US waters. A lot of industrial vat coating experience too, involving liquid products now illegal to make or sell in this county.

But some excellent options, like hypalon rubber coatings used for serious commercial roofing, can be obtained in heat-weldable roll sheet fashion instead of liquid style, and these would make a superb leakproof sink lining over plywood if you happen to know a commercial roofer with the right gear and experience. Such pros might even have a sufficient quantity of hypalon scrap sheeting leftover from some big commercial job they're willing to part with affordably. Doesn't hurt to ask. And no, you can't glue that stuff down - it's highly chemical resistant. The real deal needs professional heat welding in place.

A liner which you could sheet-cut solvent weld yourself as a good liner material over plywood would be either ABS or Sintra vinyl. These are resistant to acids, but not to strong solvents. Just this afternoon I used a leftover scrap of Sintra to make another outdoor cat house before winter. No, it's not going to win any Architectural Digest awards; but I don't listen to the complaints of tenants who never bother to pay their rent anyway.

I don't think you need marine plywood if you seal it securely. I made mine of ordinary plywood sealed with 2-part marine epoxy (West) and corners caulked (I don't recall what sort of caulk—it was too many years ago). It's still good after almost 20 years.

I forgot to mention an exhaust fan to outdoors and lightproof covering windows and doors. Safelights the appropriate distance from the enlarger and trays

Oh I like the Line-X idea. I was in a business class with a guy who owned a Line-X franchise. Some of the application examples he showed us were really thinking outside the box. Just build the sink and have them spray it- one and done!

Caulking : go marine-grade moisture-cure polyurethane like Sikaflex (maybe around ten bucks a tube these days). Avoid all acrylic caulks. Don't go silicone either because if you try to overcoat that, nothing is going to stick.

Having read all this... now I'm kinda leaning toward doing some kind of single-piece, no-seam plastic material that I can form over the plywood sink shape. I have a heat gun. Has anyone done this before? Would ABS sheet work?

BHuij said:

Having read all this... now I'm kinda leaning toward doing some kind of single-piece, no-seam plastic material that I can form over the plywood sink shape. I have a heat gun. Has anyone done this before? Would ABS sheet work?

Click to expand...

I had the local guys come in an cover my plywood sink. it cost about $200 for an L shape sink that will hold 4 20x24" trays and another 3' of space for containers, etc. I use the canadian equivalent of these guys.
https://duradek.com/utah-decks/

I suggest the use of plywood, using Marine Epoxy as the sealant for joints and to make the wood surfaces waterproof...with or without using fiberglass cloth at the joints to reinformce.
West Systems epoxy (2 parts) is frequently used in laying up wooden hulls for racing sailboats, and it withstands the pounding that hulls experience in ocean waves.

Glad to see the suggestions move in the right direction. Kind of wish I'd tried this back in the day. In those days of yore, I installed a stainless steel sink. No funny business, but it cost an arm and a leg.... which it wasn't worth, and after we sold the place, that was among the first things to go on the new owner's list (not that it matters, but it was a big, beautiful sink!). So I like the homemade approach.

But what I don't think the OP mentioned explicityly is what is intended with the sink? Will it be simply for developing negatives, or will it be for washing prints... and if the latter, how big? Sounds at 2X4 feet you could do a lot, but if you're not printing, you can get by with a lot less.

These days, I'm using a laundry sink. But my use is strictly for washing developing gear and draining our highly diluted chems while processing negatives. I'm ink printing and not trying to wash big, beautiful prints. If I were doing that, I think I'd head here if I couldn't find a place - public darkroom / art room - that had a sink first. Some day, I do hope to have access to a public darkroom (local community college hopefully) for this.... but it likely won't be close by. For me, the only reason to go this way would be to do alternative process printing (contact prints, kalilotypes, etc) from digital negatives.... which is on my bucket list, but way out there and a bit paused at the moment. Max size I can imagine having looked at the gear is I think 11 X 14. But for now, plain old ink printing is enough.

Can you provide a little more background for the motivation behind a DIY approach to this project?

Is the reason you want to build a darkroom sink primarily to save money? Or is it because you cannot find a suitable sink manufactured in the right size for your work space?

That is, are you willing to make the investment in time and materials that will result in a well engineered, long lasting sink? Or do you want to build it quick and cheap, while accepting the possibility that compromises in methods and materials might result problems a few years down the road?

BHuij said:

Having read all this... now I'm kinda leaning toward doing some kind of single-piece, no-seam plastic material that I can form over the plywood sink shape. I have a heat gun. Has anyone done this before? Would ABS sheet work?

Click to expand...

It might be possible to heat sheet plastic enough to get it to conform (more or less) to the inside of your plywood box, but I anticipate that is going to be very difficult due to the compound curves required in the corners. Some canoes are made by using a vacuum to suck heated sheets of ABS into a mold -- but that takes a big oven to heat the plastic and a lot of suction. Not practical for one-off projects.

Sheets of plastic high pressure laminates made for kitchen countertops might work. These might be called melamine - of which Formica is one brand. Rather than trying to bend the countertop material, I would be inclined to laminate the plastic to the plywood in flat sheets, then use the best waterproof caulk I can find to seal the inside joints.

As a boat builder, I would approach the project using plywood boat building materials and methods (epoxy resin and fiberglass cloth). Boatbuilding methods will be more time consuming and messy than it would be to use a sheet of high pressure laminate countertop material. No idea how well the countertop material would work in this application. But I am pretty sure the epoxy + fiberglass approach will work if you do the research to get the right products and learn how to use them properly.

If you decide to go the epoxy route, I suggest sticking with one of the reputable brands of boat building epoxies such as WEST, System 3, or MAS.

Both WEST and System 3 offer PDF books as free downloads which provide good information about how to use epoxy and wood to keep water out (or in). <System 3 epoxy book> <WEST epoxy book>

To answer some questions, the reasons I want to go the DIY route are as follows:

1. My darkroom is a very small repurposed bathroom. I have very limited space to work with, and haven't found commercial sink options that fit the bill for what I want.
2. I want to save money, and commercial sinks have all been a lot more expensive than I wanted.
3. It's fun

I want the sink primarily for putting trays in. my 2'x4' design should allow me to fit a couple of 16x20 trays in (or 3 11x14 trays). I'd love to go longer, but there just isn't space in the room. That's for standard silver gelatin printing. I will also use it for kallitype, cyanotype, salted paper, and potentially other alt processes in the future. And of course film development, B&W/C-41/ECN-2/E6. And while we're at it, if Cibachrome ever gets resurrected...

I am putting out a few feelers. I reached out to a Line-X company for a quote. I've been looking at epoxy paints available at my local hardware stores. And Home Depot has 1/16" polypropylene sheeting that seems like a decent option. I could probably just cut sheets of that to size to cover the inside of the sink, and then seal the seams with something like 5200 and call it a day.

BHuij said:

To answer some questions, the reasons I want to go the DIY route are as follows:

1. My darkroom is a very small repurposed bathroom. I have very limited space to work with, and haven't found commercial sink options that fit the bill for what I want.
2. I want to save money, and commercial sinks have all been a lot more expensive than I wanted.
3. It's fun

I want the sink primarily for putting trays in. my 2'x4' design should allow me to fit a couple of 16x20 trays in (or 3 11x14 trays). I'd love to go longer, but there just isn't space in the room. That's for standard silver gelatin printing. I will also use it for kallitype, cyanotype, salted paper, and potentially other alt processes in the future. And of course film development, B&W/C-41/ECN-2/E6. And while we're at it, if Cibachrome ever gets resurrected...

I am putting out a few feelers. I reached out to a Line-X company for a quote. I've been looking at epoxy paints available at my local hardware stores. And Home Depot has 1/16" polypropylene sheeting that seems like a decent option. I could probably just cut sheets of that to size to cover the inside of the sink, and then seal the seams with something like 5200 and call it a day.

Click to expand...

All very good reasons, especially #3!

I see the Home Depot website offers several different kinds of sheet plastic material, but am not finding any which are described as "polypropylene."

I believe polypropylene has a reputation for being difficult to glue, so you might want to look into that. The technical data sheet for <3M 5200> does not specifically say if they recommend it for use with polypropylene or not, but I notice the spec chart shows 5200 has much lower numbers for Overlap Shear Strength when used with polypropylene compared to most other materials.

So I think some kind of plastic sheet may work well as you plan to use it, but I am not sure polypropylene is the right plastic for the job(?)