Scale Soaring UK

Hi,

I am painting my SHK and have a possible issue I wonder if someone else has experienced.

The wings are covered with Diatex and then doped as per their instructions, however, I did not add the silver paint.
I am using Skycrafts coloured dope to paint. I am using an HVLP gun and have thinned the paint to 50%.

The white went on just fine and I got good coverage but then it was time for the blue underside...
Day 1
I sprayed the topside white and got a good result.

Day 2
Now it was time for the underside which was to be blue.
I sprayed the wing and thought I had put on enough to get coverage; however, in the evening I could see that the coverage was not enough.

Day 3
I now added three additional layers and thought the wing looked "wet".
In total, I might have used about 1-1.25 litre

Now when looking at the wing I can see that the coverage once again not enough.
As you can see on the images, the coverage on spars and ribs are much better.

Questions
What do you think is the problem
Have anyone experienced the same and what did you do?
What would you recommend me to do

Thanks, Lars

Lars, how did you apply the dope before the colour coats, brush or spray?

Robbie, brush and several layers

One of the problems with Diatex and other non adhesive backed covering fabrics is sealing the weave before painting. If you spray the sealing dope on, because it is thinned a lot in order to facilitate spraying, the volatile thinners flash off and push the dope out of the weave and therefore, the dope has not keyed through the weave and it will not be sealed. That's why it is important to brush the dope on as you have done to make sure it has penetrated the weave.

What isn't obvious, as I also found out to my cost in my early experiences with Diatex, you really should only thin the dope as little as is necessary to make it brushable - thus reducing the possibility of any thinners flashing the dope out of the weave. Even then, you are quite likely to have some 'pinholing' problems but nothing like you have experienced there.

It's a thin line between brushing enough dope through the weave to seal it and not getting some pooling on the inside of the weave although some full size inspectors like to see that as it is a good indicator of good weave penetration; but they don't have quite the weight considerations that we do.

You can see where the fabric is attached to the parts of the wing that have had adhesive applied to them the colour seems to have taken ok as the weave would have been adequately sealed at those points.

Would it be possible to apply another much thicker coat of dope over the paint?

Robbie,
Do you mean dope and not less thinned paint?
Lars

Yes, less thinned dope and again, brushed on.

When sealing this type of fabric I have one of Skycraft's doping brushes trimmed down a little to make it a little stiffer to force the dope through the weave - just be careful not to flood the fabric, brush it out well and try not to do too large an area with each brush load. Heaven only knows what the paint will look like after applying more dope on top but subsequent colour coats should deal with that ok - always try a small patch first!

Another thing I would have done, before applying the colour coats I would have one or two coats of grey or white primer, the last one rubbed down dry and the dust only wiped off, not blown or vacuumed. This way any remaining pinholes hopefully will remain filled.

The aluminium paint you mentioned is only a UV protection layer to protect the fabric.

I have spoken to Skycraft and they will send me a new can of paint.
They say they will add more colour and then this additive that makes it not crack later on.

We will soon see how that goes.

Ah, maybe they know something we don't..............

Yes, especially as the white paint which is supposed to be the same worked just fine…

I normally use water based clear varnish,brushed on to fill the weave,then spray with cellulose

Barrie Burton

That was clever, much better as it stops the thinner.
Do you possibly have the name of the water based varnish?

Any of the usual types found at DIY stores.Check how the brushes are cleaned(the small print under directions)if water that is OK if spirits it is not water based.You can get glossy or matt,I ust matt.Normally from B&Q or Homebase

Thanks, to the shop then

Maybe I was too quick there. Do you think the water based varnish will find grip on the nitrate dope?

Lars,did you mean dope sticking to the varnish?either way I have had no problems.I believe that all cars are now sprayed with water based paints and they have no problems with adhesion.

Barrie,
Did you sand the varnish before adding the coloured dope and on my can it says “dry three days”
Lars

Morning Lars,I dont sand,and usually leave structure outside in the sun to dry,difficult this time of year,but another plus,it does not smell,so you can dry it in the house,without the wife knowing.

Lars,I have a tin of Ronseal interior varnish,satin clear touch dry 20 mins.Wash brush in warm soapy water.

I used Diatex on my K8 and had no problem painting it. I sealed it with a 50/50 mix of nitrate and butyl dope, so a little bit of shrinakge but not too much. Like Robbie said, the trick is to thin it just the right amount so that it brushes out (use a good quality flat brush) without leaving tracks, but it filled the weave.I played around with how much thinner I added (it is also temparature dependant) until I was happy with the results. I think I gave it three coats in the end.
I sprayed 2k straight onto it and a light rub down with a 3M green pad and a good wash down.
John

Hi John,
Yes, everything went well with the white paint it seems to be something in the blue paint which causes this issue.
Skycraft have now sent me blue dye to add; however, I am doing some tests and will report back soon.
Lars

Hi Lars, just found this thread. Any update on the Diatex paint problem?

I have just finished my Habicht and the wings are covered in it and that was after a problem with Ceconite 104. Mine went fine after several brushed coats of nitrate dope, then a couple of coats of white primer, Halfords best rattle cans? Then their rattle can VW Candy White. No problems wit that. The red sunburst was done in air brushed acrylic.

I tried to brush more dope on the Ceconite after the white paint and all ~I had was a hell of a mess. So the Ceconite was stripped off and I started again with the Diatex 1000. Not something I would like to have to do again!

Hi John,
Long story short I think the problem is that the blue paint is sensitive to high temperatures.
In the autumn I did a new try and now the temperature was around 18-20 degrees.
Suddenly the paint no longer ate itself through the dope, the result was great.

After painting the wings I sanded them down using wet sandpaper, starting with 1000 and progressed down to 4000.
I learned the hard way that I had to avoid sanding in circles, keep it straight(cross) and it will be fine.

Once I had done that I took a car rub polish Meguiars Scratch X 2 and an electric drill with a polish pad.
The result was a mirror finish.
It’s worth repeating that the white paint was excellent.
Lars