I use a product called TINNIT from Datak [Rockford, Il, USA], and it works reasonably well..
It comes as two small packets of tinning salts, I dissolve one in about 6oz of hot water, and store in a plastic container. A packet will plate [supposedly] up to 300 sq in. but I don't usually get that much as I prefer a slightly heavier plating layer. shelf life is about 6 months.
The solution is designed to work best when heated to appx 130F, and takes 10 to 30 minutes depending on thickness required and pcb size...
It also requires a quick neutralizing wash with houshold ammonia.
When not heated some yellow fine crystals are in the solution, but dissolve when heated.
Thier part number is ER18
personally I prefer a solder bath / roller application, it's quite a bit heavier, can allow higher trace current, plus I like the 'wrinkles' on a non leveled application, looks nice when conformal coated...