I've used Fullerplast for quite a while, I think I bought my first gallon from Fuller Obrien 20 years ago. I was told by the guys at Van-dee's that they bought the rights to the product and that they were the only place to get it and it only comes in gallons. The last time I ordered it I think that it came to about $90 with shipping.
I spray it with a air brush. I thin all of the coats because it sprays a lot better that way but I especially thin the first coat so that it will soak in as much as possible.
I don't think thinning it hurts the application at all, if anything it gives you a better thinner coat. I will ussually sand it pretty good after the first coat and maybe even after the second coat to try to fill in the pores.
a couple things, when you decide to spay a bow be sure to set aside enough time to be able to spay all of your coats within 3 or 4 hours of each other or prefferably sooner. The only problem I've ever had with the stuff is when I waited to long to spray the last coat and the prior coat had dried to much to allow for a good bond.
and some guys buy two bottles of catalyst and use twice as much as is called for. It's supposed to make it dry even faster so you don't have to wait as long between coats, but it dries so fast as it is, I've never thought that was necessary.
Unless you build a lot of bows you will never use a gallon before it goes bad on you. I think I sprayed about 30 bows and still had a half gallon left over when I finally had to replace it.