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spar varnish vs polyurathane

Posted By:
Mike Edwards
Homebuilder or Craftsman
10
Posts
1
#1 Posted: 8/12/2010 07:13:37

I am rebuilding my Baby Ace the old plans called for spar varnish. can I use poly urathane just as well? Mike Edwards EAA 45432



Chris Aerts
30
Posts
4
#2 Posted: 8/12/2010 07:36:20

Mike spar varnish is normally exterior, poly is normally interior.



Mike Edwards
Homebuilder or Craftsman
10
Posts
1
#3 Posted: 8/12/2010 14:35:23

Thanks Chris that helps a lot ...Mike



Bruce Rhymes
Vintage Aircraft Association Member
1
Post
1
#4 Posted: 8/12/2010 22:03:31

Mike:

I'm a long-time woodworker, and built my own sailboat.  I maintained the varnish fanatically, and also did some varnish brushwork for hire when I was cruising on her...

Modern spar varnishes and polyurethanes are remarkably durable, and almost all the quality finishes employ UV inhibitors.

If your finished surface will never be in direct sunlight, then the durability of polyurethane will make it the right choice.  If exterior woodwork is to be finished, choose spar varnish. 

Here's the reason for my bias:  Clear finishes succumb eventually to ultraviolet light, much more so than through wear and tear.  Eventually, even the most well-maintained clear finish will have to be stripped and taken to bare wood, (although this period may well be many years.)  Polyurethanes are very difficult to strip, and could possibly require the use of chemical strippers... while good ol' spar varnish can be removed with a sharp scraper and some sanding.   Either finish would need re-coating at the same time, anyhow.

One more point:  Build up your coats with gloss finishes, sanding between each coat with 220 grit or finer.   Then, for your final coat use the satin, eggshell, etc. for the look you want.  This is because the gloss finishes are waterproof, while the dulled finishes are necessarily porous. 

How many coats?  Glad you asked!   I can still hear the old British shipwright saying:  "Apply seven coats.  By seven coats, I do not mean six coats!"   Did I always follow his advice?   I'll never tell!

Bruce Rhymes

EAA#552834

 



Mike Edwards
Homebuilder or Craftsman
10
Posts
1
#5 Posted: 8/13/2010 05:50:50

Bruce ... thank you so much for the advice. I learnd a bunch from it. I think I will continue on with the spar varnish. 7 coats not 6 ...Mike



Anne McCombs
2
Posts
2
#6 Posted: 8/13/2010 12:02:30

One comment in the "you probably already know this" category:  Make sure you use dope-proof varnish on any wood that touches fabric, such as rib cap strips. 

 

Anne McCombs



Jonathan Tibbets
Young Eagles Pilot or VolunteerHomebuilder or Craftsman
7
Posts
0
#7 Posted: 8/25/2010 08:37:42

I also am restoring a Corben Baby Ace. An important item to keep in mind is the type of covering system you are planning to use. PolyFiber coatings will attack any single part varnish. Your safest route is to choose a two-part epoxy varnish to protect your wood structure. Whichever one you choose, it should be epoxy, rather than a vinylester coating.

Jonathan Tibbets

EAA 299851



Mike Edwards
Homebuilder or Craftsman
10
Posts
1
#8 Posted: 8/25/2010 19:29:49

I hope I'm not in trouble here..I have already varnished the "birdcage" and getting ready to install it on the frame. What kind of covering do you suggest other than polyfiber.. that is what I am used to. I might get a two part and go over the top of the single part Spar varnish to form a barrier.  thanks for your help...Mike



Jonathan Tibbets
Young Eagles Pilot or VolunteerHomebuilder or Craftsman
7
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0
#9 Posted: 9/8/2010 17:35:15

As long as there is some "tooth" on the one-part varnish (not glass-smooth) the two-part epoxy will work just fine.



Larry Martin
12
Posts
0
#10 Posted: 9/10/2010 18:01:42

Use "Stewart Systems", it water based glue....

 

  loved it on a Banty!



Kenneth Phail
Homebuilder or Craftsman
14
Posts
3
#11 Posted: 4/26/2011 07:27:11

Mike, I used spar varnish on my wood airframe -but discovered the problem with covering systems lifting the varnish in time. Fortunately you can use an epoxy varnish over the spar just on those areas where the cloth will contact. Sounded like a lot of work, but it's only a very small portion of the airframe. ( I did test the varnish with the covering chemicals and sure enough it lifted it--so don't even try it !!! ) Ken