I keep getting these hairy spots.  The wing is covered with polyspan, had spots where I went through and had patched with silkspan and Brodak rejuvinator (the entire finish is Brodak).  What did I do wrong and what is the fix?

Thanks,

James

I am simply not skilled enough to sand a clear filler coat without sanding down through the base primer underneath.  The only way that I can reliably avoid sanding through is to have a base of at least a slightly contrasting color, and pigmented primer.  As soon as I see the last of the primer feathering away, I stop (well, at least I try -- then curse myself if I give in and sand "just a bit more").  Then I put on another coat of filler, and start sanding again.

The idea of fill/sand is that you build up everything, then sand down the high spots to the base material -- if you're sanding into the base material then you've gone too far, and (as you're finding) you may well have to do a bunch of repair just to get yourself back to where you were before you sanded that spot "just a bit more".

I haven't been painting with dope for ages, but I'd seal the polyspan with clear, put on a coat of whatever dark I had lying around (maybe mixed with clear -- it doesn't need to be pretty, just dark), then use Brodak's gray primer for the filler.  Then, any time I started seeing dark, I'd stop sanding that spot.

I take it you are referring to the Polyspan Fuzzies.  Pete Peterson, master of Polyspan, to whom I am but knee high, told me to fear not the Fuzzies, but to give them a toot of clear and then sand with fine sandpaper.  I found that 1000-grit works a treat. 

 It's also possible that you have put down the Polyspan with the wrong side up. If so, it will be almost impossible to sand without getting fuzzies and will take a LOT of extra clear before any sanding to help the problem. It might even be a better decision to just peel it up and start over if you have attached it upside down.

 Also, if your "hand is too heavy" to sand things without sanding through, try using a lesser grit to give yourself a chance to catch the problem earlier. Always think patience and finesse when sanding, and stop more often than you are to check progress.

It's also possible that you have put down the Polyspan with the wrong side up. If so, it will be almost impossible to sand without getting fuzzies and will take a LOT of extra clear before any sanding to help the problem. It might even be a better decision to just peel it up and start over if you have attached it upside down.

 Also, if your "hand is too heavy" to sand things without sanding through, try using a lesser grit to give yourself a chance to catch the problem earlier. Always think patience and finesse when sanding, and stop more often than you are to check progress.

I did make sure that I had the correct side up, I got the stuff from Tom Morris and doubled checked with him before I procedded.  I think my problem is that I still haven't learned to have a soft touch with the sand paper.

Thanks,

James

That is my favorite thing about Polyspan so far: it's not easy to sand through on edges.  The Fuzzies are a much lesser problem.  I used PT Granderson's* finishing system and put some dark blue in with the talc and clear dope filler.  The result acted as a guide coat, I guess.  The Fuzzies, when they appeared, were a light blue color quite different from the rest of the filler.  That's when I knew to stop sanding and put a dab of clear on the fuzzy area.

*of the Jive Combat Team

Randy : Where can I buy some of this "practice" you speak of?

HI James,

What grit were you using?

Thanks
Bill

Don't use  grit !!

 

Randy

I have used quite a bit of Poly-span in the last few years.   I used when I got my first roll at VSC many years ago before the gentleman let it go commercial.   Even a copy of the Flying Models article was given me at the time.   Sparky put me onto this,  two coats of dope, clear, then let it set for a day.   Light, very light sanding and then two more coats of clear dope.   When the dope starts to get a sheen to it, then think talc and dope it you are doing a color  finish.   On my latest plane which will stay clear I now have 10 coats of clear.  By the way I use butyrate from bare wood on to final coat.   

Bill, I started with 400 and went to 600.  I think part of the problem is I needed more clear dope after I put the polyspan on the wing and silkspan on the rest.  Still lots to learn.  I've made some progress, have it down to just a few "hairy" spots.

James

Hi James,

Yep, I would say that you simply need more dope on the surface before you begin to sand on it.

Bill