Thanks for the input Tett.

I too have used the Penetrol but probably was not mixing enough in. Probably 10% or so and then some mineral spirits. I was a little hesitant to mix more than 10% of anything due to the directions on the can of Brightside, but 75% mix with mineral spirits seems to work best.

My biggest concern was lack of gloss or durability by thinning the paint so much, but after a good wet sanding it seems to be holding on very well.