# Sealing the wooden airframe......

Discussion in 'Finishing Techniques' started by MadProfessor8138, Jan 3, 2019.

1. Jan 5, 2019

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8. Jan 8, 2019

### proppastie

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Is the PolyFiber Poly-Tone a finish, will it protect the wood? So why the Epoxy?

9. Jan 8, 2019

### TFF

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Except for the urethane, Polyfiber products all start with a the same base stock. Pink stuff, polybrush, can also be bought clear, Polyspray is just aluminum flakes added to the polybrush. Polytone is color pigment and something for weathering. They all melt together. Every time I use the pink stuff I think some poor Naugahyde chair from the 70's was melted down to make it. It's just simple evaporating paint. Epoxy does the chemical change thing that is leaps above air dry paint. That is why the urethane is so good, bulletproof to most chemicals; just hard to repair small damage.

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10. Jan 8, 2019

### FritzW

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Poly-Tone dissolves in MEK so the Poly-Tac wouldn't work.

11. Jan 9, 2019

### proppastie

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was thinking after it is all done.....no epoxy...glue and finish the plane over the bare wood, the initial prep and Poly-Tone final coat is a finish that might seal/protect the wood?......less steps, less weight?

12. Jan 9, 2019

### FritzW

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If your just sealing and painting wood that's not going to have fabric over it you could use polyurethane for a sealer/filler and spray Poly-Tone over it. I wouldn't try to seal wood with something that wasn't specifically designed to be a sealer (ie. Poly-Tac, Poly-Spray or Poly-Tone).

13. Jan 11, 2019

### Rockiedog2

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Lotsa experience and good advise in the replies

I used to use spar varnish way back before I discovered Poly Fiber EV400; that's the only thing I use now. Pic below of my Acrosport wing that was damaged and had to be rebuilt

Aircraft grade hardware installed inside the AL leading edge, about 6 years old. Rusty and corroded. Hangared. The varnish was spar varnish, I think Red Devil or one of the hardware store brands. Dunno what caused the corrosion but highly suspect the varnish; what else could it be? So went to EV400 and haven't found anything that beats it. Thin enough so it can be rubbed in and durable enough I don't use but one coat, very light. The + is nothing vs the project cost. T88 will go over it and not attack it and will give a good bond if the EV is sanded and blown clean of all dust, it will be a mechanical bond.
Random thoughts I learned over the years, some of it just personal technique that works for me
I try not to mix types/brands of coatings like spar, poly, epoxy varnish, Poly Fiber, Randolph etc etc. Sometimes can't be helped like when doing repairs but for scratch built I am able to preplan it so don't mix anything. All I use is Polyfiber given a choice. except I like Randolph epoxy primer on tube airframes. Military green versus the funky PolyFiber light green. Back in the old days zinc chromate primer and when Polytac came along it would attack it and if fabric was applied over it would likely rust inside the fabric. Epoxy only.
Polytac can be applied over EV400 as already mentioned several times and should be sanded first and all dust removed especially inportant when gluing fabric to rib caps as in the Eagles. If not sanded there will be little nibs that will hold the fabric off the cap and prevent a good bond. Often I scrape a particularly important glue area like the rib caps. Poly Fiber told me it's ok to Polytac over Polybrush(like on a leading edge) but I don't like that and don't do it. Where a fabric seam will be I mask off for the polytac before polybrush applied. Often the seam against the wood isn't structural, the fabric to fabric seam is the structural one so the fabric to wood seam doesn't matter much when the fabric to fabric is done. Inside the leading edge I varnish up to where the noseblock will be and when gluing the noseblock in, use T88 to seal the area that didn't get EV400. Some of the above obvious I know.
T88 tech says that when repairing, for instance, that it's ok to glue over an old T88 seam with new T88. I don't like that as the new T88 doesn't soften the old T88 like the PF products do to one another(they chemical blend) rather than just stick to the top of one another but not so new T88 over old T88, that will be purely a mechanical bond so I take particular care to true up and roughen that joint area so the new T88 will have a tooth to grab onto.

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14. Jan 11, 2019

### FritzW

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I'm just guessing but I'd bet dollars to dog turds that that's galvanic corrosion and the varnish didn't have anything (or much) to do with it.

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15. Jan 11, 2019

### lr27

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If you bet "dollars to dog turds", you lose no matter what. ;-p

16. Jan 11, 2019

### Rockiedog2

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Well, I dunno Fritz. It was only inside the leading edge and random there and all new AN hardware when installed and the backside of the strap bolts wasn't corroded. I haven't seen it before or since...only thing I could figure was something to do with varnish fumes (or something) in that more or less sealed space. Yeh I know that doesn't make sense but nothing else I could think of does either. I won't argue it either way...

17. Jan 11, 2019

### proppastie

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would that area of the LE be relatively "air tight" and cause condensation to the colder metal parts?....was the threaded rod SS?

18. Jan 11, 2019

### FritzW

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Like I said, I'm just guessing. But when you see two parts fastened together and one is corroded (anode) like the nuts on the drag rod and the other isn't corroded (cathode) like the drag rod itself. It makes me think it's galvanic corrosion. (same thing with the strap and strap bolts).

...that's not to say the varnish fumes trapped in the LE weren't the electrolyte that made it all happen.

EDIT: Either way it's a good argument for ventilation and drain holes.

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19. Jan 11, 2019

### TFF

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I would lay the bet that it did not stick to the hardware and instead humidity was able to condensate and sit on it. Unless its oil , epoxy is going to stick to anything. If you read the The cans for hardware store spar varnish, they all recomened limited outdoor exposure. How much better the stuff is compared to something from the 50's is probably subjective. Real spar varnish on boats was reapplied every year and the new coat softened the old coat. Urethane will make a nice solid surface but recoat is mechanical bond. I have some hardware store varnish on some moveable ply decking. It's outside for days at a time and has checked surface. About two years old from new. Epoxy mechanical bond is pretty much the best any one of us can get a hold of. If the surface is going to buckle and open up, you can't stop that without some other outside covering; glue in any form is not going to stop it. More varnish is about filling the new cracks. Back when planes were covered with cotton, it came off every five years and everything got revarnished. Today with 30 year coverings, you can't rely on catching it at the next covering job.

20. Jan 11, 2019

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