Try to do it right w/o sizing first. Sizing is an extra complication and may not be needed with some papers. First see if you're able to get smooth tonality then you'll go for contrast and sharpness (by sizing) if you need it... AFAIK, Arches Platine has a fairly stong size already.
BTW, what was the strength of sizing? Gritty surface hints too strong sizing solution for the bloom of your gelatin. Also, tray sizing is a mess; if you absolutely need to size then do it with a foam brush, working the size in paper thorougly, keeping the gelatin/salt solution in a vessel that stays on a hot plate meanwhile.
Is your environment too dry? Excessively dry (< 40% RH) environment may cause uneven sensitizing. Try to raise the RH and/or use an ultrasonic humidifier to humidify the paper (by keeping it in front of the nozzle).
4-5ml per 5x7 seems ultra excessive to me. I'd use less then 1 ml for that size if coating with a rod.