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Lightwight foam cowl, Tissue with EZE Dope or thinned PVA?

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martin collins 102/07/2020 17:11:19
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I have made a hollowed out white packing foam cowl for my Jodel project and need to finish it so it can be painted, options are EZE Dope and tissue or thinned PVA and tissue. Bit concerned the EZE dope might distort the cowl as it shrinks, would probably have to thin it, what ratio? Opinions needed as it has to be done this evening as i`m on a time scheduke with this project. Many thanks...........

kc02/07/2020 17:34:14
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Chris Golds wrote an article years ago describing using foam covered in brown paper and dilute PVA. He used thin brown paper which is slightly shiny one side. This worked well when I tried it on some components such as cockpit headrest. Came out hard and like it was carved in balsa but lighter. Suggest you use brown paper.

kc02/07/2020 17:36:26
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One of my clubmates showed me his models covered in coloured parcel tape - looked a good idea if there are only curves in one plane.

martin collins 102/07/2020 17:46:30
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Quite a sharp curve on the bottom of the cowl hence the idea of using tissue.........

Erfolg02/07/2020 17:51:51
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The foam type may matter. Some things do not like all foams. Dope can be one, cyno another.

I tend to use surface glass cloth, as it drapes well in all directions. More often than not I use WBV (water based varnish), if I just want to resist minor dings. If I am looking for strength, I would use Epoxy Resin, although heavier and a heavier lay up.

I have used brown paper parcel tape, works well on simple curves, and can be very tough. Vic Smeed even made model boat hulls out of it. Not particularly light weight, if weight is an issue.

A lot depends on what you have to hand. An example is parcel tape (paper) once freely available and as cheap as chips. Now a lot depends from where you buy it (if necessary).

Simon Chaddock02/07/2020 18:05:20
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PVA and tissue on foam is safe enough. I done it several times. The quality of the tissue can make all the difference as to how easy it will be to do! wink 2.

Erfolg03/07/2020 11:53:36
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Picking up on Simon's option, making the assumption that traditional modeling tissue is not to hand, then Man Strength Tissue paper hankies of the disposable type, are usable. Not quite as strong or necessarily as easy to work, they will work, may be requiring more layers and care on application.

I have tries the tissue paper of the type that comes as packing with shirts. I found that it is more akin to the Japanese suff, in that it is much harder, also does not accept dope (probably a no, no with foam) as well, although sticks with PVA.

I have tried the type of tissue that is often found in low cost card shops, some is like the stuff that comes with shirts, other similar to model aircraft types. It all depends, it seems. Very cheap though, typically about £1 for a few sheets.

In recent times I have good experience with Newspaper and wall paper glue (cellose). I have made a prehistoric cave and camp, a Viking helmet, and a "rain cycle).

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There is a lot that can be done with scrap materials. Living at the end of the road, I am increasingly learning how to live what can be found around the house and the local non specialist shops.

Bruce Collinson03/07/2020 13:22:46
543 forum posts

Re KC and Erf's references to brown paper, that's what Richard Wills advocates for his Warbird Replicas, of which much has been written here in the last month. Soak for 10 mins in slightly diluted pva, allegedly goes round compound curves.

BTC

kc03/07/2020 13:37:04
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Chris Gold's method seems different ( he does NOT soak IN the pva) he used 70 percent PVA and 30 percent water, pasted both the brown paper and the foam surface( using brush I suppose) then applied quickly BEFORE the pva soaked to the outer surface of the brown paper. Cuts were made in the brown paper to allow it to fit curves. Thats what i did too.

Easy enough to experiment with either way on a test piece although Martin won't have the time to test first due to his time schedule.

Erfolg03/07/2020 15:34:54
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KC

I expect that it is a case of "there is more than one way to skin a cat".

I can see the logic of soaking for some time, in that the fibres making up the drown paper will soften, or probably soften. How much of the PVA will soak into the paper, will depend on the paper primarily. I can see benefit in getting PVA into the paper, to toughen and harden the material.

Personally, now, I would not dismiss Newspaper as a material. That is mainly due to my work with my granddaughter (no. 3), where papier mache produced a surprisingly strong result, at low weight (although weight was not an issue). Recently I did use the method to produce a cowl I think. I did not use it opting for another solution. The bottom line is that it does work, if done properly. I am still racking what is left of my brain as to the intended application.

Edited By Erfolg on 03/07/2020 15:36:32

Dad_flyer04/07/2020 00:10:02
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From Martin's original post, there is no particular worry about EZE dope distorting the cowl by shrinkage. In the EZE dope method you water shrink first, then seal with the EZE dope. It is water based, but has no special shrinking properties. I would imagine shrinkage would be similar to thinned PVA, and just from the water in both.

ron evans04/07/2020 11:49:42
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22 photos

+ 1 for glass cloth and WBP. I've used it on a few models with different types of foam, and it works really well.

This hollowed out blue foam fuz was quite flimsy before glass cloth and Wilko varnish, and it takes compound curves easily. The weave is then filled with a varnish/lightweight filler mix and light sanding in between. Balsa wing finished with the same mix.

Not tried newspaper with varnish yet.....should you use the Mirror for the left wing and the Sun for the right ? smileyp9162818.jpg

Ron

Edited By ron evans on 04/07/2020 11:50:58

Erfolg05/07/2020 14:08:05
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Whem you come to finish your model a few pictures to help.

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I trust yours will not be gutless. Is it an own design?

I increasingly use WBV on foam with cloth.

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The wing tips on this are blue foam with WBV, in this case Ronseal Hardglaze, which is a bit harder than Wickes or Toolstation, at a much higher cost. Is it worth it? Only if you have some left over from an other job.

I have used tissue paper on Blue Foam, for me not good. The model below is a combination of WBV and glass and tissue paper and PVA.

This is what it should look like. In my model it has never been finished as a suitable power plant has not been found to provide an acceptable CG.

I increasingly take the view, that most systems do work, if care is taken. More skilled people than me will achieve good finish with light weight.

ron evans05/07/2020 14:54:33
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Hi Erfolg, thanks for the pics, but the model was finished and flown some time back. See my photos.

I too seem to be using WBP varnish more and more. my last build in the lockdown used it with coloured tissue on balsa. A nice hard finish and easy cleanup.

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