Over the years I have found that a foam brush tends to make bubbles as you use it. Personally I just use them for staining.Being that your varnish is fairly thick it makes me think your bubbles are not able to pop,thus showing up when the coat has dried.It could also be what I call gassing.That is where the solvents in the previous coat haven't had a chance to get out .How long are you waiting between coats?I usually wait at least a day between coats when using waterbased polys,and even 2 days with oil or solvent based products.Maybe you could try and spray it with a cup gun like the ones Wagner makes.Iv'e used them before,they are pretty handy for some stuff.I use a Wagner airless paint sprayer myself.As the other fellas said,filter your materials, use a tack cloth and don't raise any dust if possible. By the way,the bed looks really nice.
48 Ford F4 flat 6/4 spd,dump bed
92 Ford F150XLT 4x4 ,5.0 E4OD
98 Ford F150 XLT 4X4 5.4