What would cause polyurethane floor varnish to dry with tiny pimply bubbles in it?

It could be lot's of different things but if the floor is clean...Preparation is possibly wrong, wrong type of brush can cause air bubbles, so can very dry timber, damp timber, certain timbers react to the chemicals in certain varnishes, stirring the varnish up too much can cause bubbles...not brushing it in well enough...lot's of thin coats sanded between coats is better than a couple of thicker coats. I would say thin the varnish right down so it absorbs better and see how it goes

have you tried sanding between coats? we do it on every job as it doesnt matter how well you sand the wood the laqour will dry rough/prickley on the first coat. first apply 2 coats then you need to use a light grade paper 100 g up just rub over by hand gently until any contamination is out and the floor feels smoothe then apply the third coat. if you have tried this and it doesnt work try junkers base prime as a base coat this is quite agresive and will take well to pretty much anything. and if this doesnt work try bona or junkers polyurethane laqours you shouldnt have any problems

I tend to go with Restofloors with this one, I would want to know the history of the floor, for example what was used previously on the floor. Wax or Oil contaminants can sit in the base of the board and appear only when the outer surface is sealed causing the contaminant to rise to the surface area. Definitely go with a primer first before tackling the finish. Hope this helps......