New covering methods for EPP

[GotrekGurnisson]

I think my problem may have been too much water. Vise doesn't willingly mix with water. Might try it without water next time.

[thevon]

I've just found out that on RC Groups there was a huge discussion commenced in 2005 about this method of mixing PU with spackle.

www.rcgroups.com/forums/showthread.php?t=422127

It's so long, I read a lot of it but it's just too big! Anyway the summary is they had a lot of guys around the world doing experiments and although there were some interesting alternatives (such as epoxy and microballoons;, different types of lacquer) the outcome is most guys agreed that mixing PU with lightweight spackle was a huge improvement over plain spackle and easy to do. If you mix 10% PU it helps and if you mix 50% it bonds lots better but the working time is so short you can only do a bit at a time.

It seems the recommended method was:
PU/spackle, dry and sand.
A layer of thinned Goop (Shoe-Goo) and let dry.Try to get the PU spackle as smooth as possible, then you need very little goop.
Fibre Tape as usual, sand the tape.
Coating of thinned Goop over the tape and let dry.
Covering of choice, preferably Profilm.

[thevon]

As said in the Bat thread, when I was ripping the covering and tape off preparing to re-cover it, on the areas where I sprayed 3M90 (near the LE's, tips) the covering has stuck MUCH harder than the other areas. The main parts where I sprayed with super 77, it's easy to pull off but it's really well bonded to the 90 areas.

I've just been spackling it and found a difference between spackles. When I used Selleys Rapid lightweight filler, it went "gummy" very quickly, so there was only about 30 seconds working time. If you touched after it went gummy it lifted off the EPP skin, but when it set, it was really well stuck and it sanded well. I used about 30% PU.

It didn't seem to be how I remembered it from before, so I got some Bostik Kwikset lightweight filler, (the first one I ever tried) and compared them head to head. The Bostik gave quite a lot more working time, and stayed creamy for longer. Even when it started to go gummy it was more workable. But when I went to sand it, it was darn tough to sand! It's cheaper too.

So there are obviously differences between fillers. If you use the Selleys, you need to be quick and only do a bit at a time. With the Bostik, don't use too much PU, the job will be easier but you need to be more careful not to build it up too thick.

[mtmellum]

thevon said:

Well guys, the results of my latest experiment were very convincing! Suffice to say, there's no way I'll ever put on spackle WITHOUT PU mixed with it!

One BIG finding is that if you goop the surface first, then spackle, you TOTALLY lose the rigidity benefit from the spackle!

A clue led me to wonder if mixing spackle with PU glue was possible, so I got an experiment going with 4 squares of EPP spackled both sides and spatula'ed smooth; the 4 treatments ranging from plain spackle, to a bit of PU, then more; then more again! The maximum was probably about 25% PU I think.

It mixes together extremely well, easier to apply than plain spackle. It didn't seem to foam at all. When dry it sands easily.

Today I sanded them and did 3 tests:
1) compared the rigidity (subjective hand-flex test!)
2)the "ripoff test" - spray gluing , sticking a bit of strapping tape on, rubbing it in hard, and ripping it off;
3) and the "crush test!"- crunch it up and see what the surface looks like.

The main points:

Stiffness was a bit better for the PU, but not hugely better. Plain spackle is pretty stiff, its main downfall is that it crushes up when damaged. The PU plasticises it and holds it together.

But when you flex it, it doesn't crack up on the surface.

The PU spackle coat doesn't powder up when you crush the surface.

When tape ripped off, this is where you can see an obvious difference and a big benefit. For the PU mix the tape ripped off leaving very little damage to the surface whereas plain spackle lifts out and lifts out bits of EPP.

excuse the edits Andrew, what ended up being the ideal ratio of PU to spackle?
thanks
dave

[thevon]

I still guesstimate it, but it's probably about 20%. Just enough to start giving it a bit of yellow colour. But it's a bit of a knack - you need to spread it really smooth with a spatula/ credit card etc, so you don't have much to sand. You can use more PU and get a tougher skin, if you skim it flat. Work along the wing and you might get 1/3 to 1/2 done and as soon as it goes a bit gooey, stop and make up a new batch and keep working along. Last time I ended up making 5 smallish mixes, to do the entire Reaper wing. But it's well worth doing, you can really see and feel the difference.

If you want it to be rubbery and tough, apply Goop over the spackle. But if you're after max rigidity, put tape layers over the spackle. When you put goop on the PU spackle it loses it stiffness and goes rubbery.

[Vanders]

Would you reccomend the 3M90 adhesive between the spackle/PU & fibre tape, as well as under the Profilm?

[thevon]

This whole thing has been evolving as we've been trying new things and learning from the Yanks ... and most of all since I've had a crack at Gooping.

No, I don't think the 3M 90 is needed between the PU/Spackle and the tape. The tape stics really well to it. And the heat film coverings (Profilm, Monokote) stick incredibly well to Goop. So the 3M90 is pretty much obsolete really. It's a great glue, much stronger than Super 77, but it's hard to spread and I can't see a lot of point in using it now.