Polyurethane Streaking Problem (part two)

by alan on January 18, 2010

This is a post I answered the other day at (Do it yourself.com). The contributor seemed to think it is a good idea to dilute polyurethane varnish by 50%. This is  to avoid what they call streaks in the U.S. but, what we call runs (U.K.)
Here you’ll find my two ha’ppence*:
Quote:
Originally Posted by aoliver View Post
My understanding from the original question was that a problem was found streaking on coats that had already been put on. My answer is that I don’t think thinning up to 50% is a good idea.

Sometimes you will thin up to 10% as a sealing coat.

Carry on doing it your way, I don’t mind!
The OP (original post – my note) streaking was caused by brushing on a thick poly, period. You can not apply it evenly, it doesn’t level as fast, thus the streaks. That’s why furniture mfgs spray and they use lacquer. (it dries fast and they can box it and send it out quickly).
His streaks were minimized by successive coats of wiped on finish. It tended to level the finish across the entire table.

Op, try your next piece with a thinned or wipe-on poly after sanding. Steel wool between coats to eliminat dust ‘nubs’. You will be more satisfied. No matter what the thinning ratio, thinner will level faster, the spirits will evaporate, and whats remaing will be a leveler finish. It just takes more coats if thinned!!!
(never use steel wool with waterbase poly).

fred
(I don’t know what streaks are, since i always thin my poly)

My Reply
In the U.K streaking means there are streaks. Differences in sheen between brush strokes, this is more likely to be due to poor preperation. We use the word runs as in tears for your streaks! When the coating is applied keeping a wet edge – the work is then gone over with a lightly dampened brush with the same finish. This is called tipping no doubt it’s called something else over your side of the water!

I would think that diluting/thinning the varnish to the degree you suggest would adversly affect the balance between solid content and solvent and be detrimental to the quality of the coat.

As to using wire wool to take off imperfections (we call denibbing), I would use a zinc stearate paper 320 or 400 between coats. If the surface is flat use a block.

*thoughts.
P.S Something tells me this disagreement will run and run!




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