Dope on Foam Core Balsa Skins

[Casey]

Has anyone tried applying dope on foam core balsa skins bearing in mind that if the dope manages to seep through the balsa skin, it will attack the foam or are there alternatives to sealing and stiffing foam core balsa skins.

[Perry Rose]

Use Min Wax Polycrylic clear.  The balsa skin is stiff enough as it is.

[Randy Cuberly]

Has anyone tried applying dope on foam core balsa skins bearing in mind that if the dope manages to seep through the balsa skin, it will attack the foam or are there alternatives to sealing and stiffing foam core balsa skins.

I've used dope on balsa skinned foam wings dozens of times as have many, many other people without problems.  I always the first couple of coats of clear straight from the can with no thinner added.  I'm not sure that's necessary but is a little extra insurance against "seeping through".  As for stiffening the balsa I cover the balsa skins with light weight silkspan after the first three coats of dope (using 50-50 thinned dope for this).  A lot of guys use Carbon Veil for the initial covering which is a lot stiffer but I don't like working with the stuff, and the silkspan has pretty much been a standard since foam wings came into being!

At any rate of the skins are properly applied you don't really have to worry about dope seeping through and destroying the foam.

My humble opinion after doing this stuff for about 53 years.  I'm sure a lot of folks agree with me.  There's undoubtedly some who don't.

Randy Cuberly

[Fredvon4]

 And to What Randy said-- I will add:

I have built a crap load of foam wings for combat planes==== some I desired a solid no pop corn finish: I glued down paper on the foam with thinned white glue....later to use Sig dope n ZS filler, then Dope color, and finally Dope clear.... in the very small areas where there was some seepage into the foam core the void was exceedingly small and did not concern me any

Only way I know about the dope voids was after crash NTSB investigation team (me) tear down analysis

[Casey]

Thanks guys for replying

[Brett Buck]

Has anyone tried applying dope on foam core balsa skins bearing in mind that if the dope manages to seep through the balsa skin, it will attack the foam or are there alternatives to sealing and stiffing foam core balsa skins.

  It seems to work just fine - as long as the sheeting is applied with epoxy as is modern practice. Back when people were using contact cement (like Home Safe, Southern Sorghum, and particularly 3M77) there was a theory that excessively thin dope was the cause for some of the problems with delamination.  I was never convinced that dope had anything to do with it, more like 3M77 couldn't stand up to sitting in a hot car all day, but it was never proven either way.

    I have never seen a problem with delamination using epoxy and I imagine that dope will get through it in places and eat some of the foam, but doesn't cause any sort of problem. I seal all the pinholes in the skins before I apply the skins.

      Brett

[Air Ministry .]

are there alternatives to sealing and stiffing foam core balsa skins.

Back in the day ?? they used to attack them with 2-pot & a bank  or playing card AND Paper Towels . No Dope . No Tissue .

Bob Hunts Genisis articals and maybe the spectrum go into the details . Dunno if its current as far as durability goes .

thus : http://www.outerzone.co.uk/planfile_supplement/7572/Genesis-1_FM_CL_oz7572_article.pdf

here http://www.outerzone.co.uk/oldpages/download_this_plan.asp?ID=7572

maybe the genisis .46 ?

Probly his Crossfire or suchlike have an updated version , 2 pot without the dope ??

pretty sure it was polyester , flatted off , primed & painted . Notes on fillets and suchlike .
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o.k. , heres somat on the Crossfire EXTREME ! , http://www.modelaviationdigital.com/modelaviation/201211?pg=131#pg131

looks like youll need to look up the previous or original whatsit to gett full info .
Remember Foam sanding blocks . Chopped from free styrene chilli bins from fruiteers or supermarket , if you havnt got foam up to the eyebrows . And Sanding Planks , Like 4 x 1/2 x 12 in & longer , From the building supplys skip bin . 

[Rich Perry]

You can use light fiberglass and finish cure epoxy resin.  That is how I finish all of my solid wings now.