coating the paper with oxalic acid seems (to me at least) a very inconsistent means of acidifying paper such as Fabriano Artistico.
The intent of an oxalic acid prebath is to neutralize the buffering agent used in paper making.
Most are buffered with calcium carbonate.
Coating the sheet of paper with a brush dipped in oxalic acid seems like it would inconsistently neutralize this buffering agent.
I would highly recommend that you completely immerse the paper in a bath of 5% - 8% oxalic acid. My normal procedure with Fabriano Artistico (Extra White is the one I use).. is to pre-bath usually 20-40 sheets at a time.
I cut the 22x30 sheets in half to make 15x22 sheets. In a large grout mixing tray I make up a big ol huge bath of 7.5% Oxalic acid. usually in the range of 8L +/- and then put each sheet in one by one until all 40 are in there. Then I'll usually shuffle from bottom to top a couple rounds (usually 4-5) then pull the out and hang them to dry.
Makes for nice consistent results.