Help sought re fine extruded polystyrene edges

[AlanRM]

Afternoon all.

 

I have created gentle contours on a very rigid (1 inch thick) ply board using extruded polystyrene (see pic), and plan to top this with Sculptamold (as per many reccs here and Dave Frary’s book).

 

The layers are bonded with an extensive grid of beads of a “Hard as Nails” equivalent (again recc by Dave Frary), applied using a caulking gun mechanism. Test pieces have shown this to be highly effective, but it leaves a very slight gap between the layers where there is no beading.

 

I haven’t used EP before so would like to ask:

 

-        Should I sand the surface prior to covering (I would rather not, as it’s more work and logic says it might even provide more grip for the Scultamold)?

-        Over time, will the fine/ “feathered” edges (some of which have a tiny air gap where there is no beading) try to flex/ move/ curl/ rise etc. beneath any covering?

-        Will Sculptamold help reinforce these edges – and, if so, how thick would I need to go?

 

All thoughts welcome and thanks in anticipation!

[HLT 0109]

I have never used Sculptamold but I have used EP extensively.  I have always used undiluted PVA to stick layers together or EvoStik wood glue and they leave no discernable gap.  Then I always cover the EP with wetted plaster bandage  and that can be used to fill or conceal any gaps.  However, a sloppy mix of plaster or DIY filler also works and will fill the gaps you have provided the mix is fairly stiff but still brushable.  It could also be applied to the gaps with somehting like a a wallpaper scraper if practice suggests it needs to be thicker than possible with a brush.

 

I certainly would NOT try to sand the edges - they will probably break up.  And I think, if left unprotected, the edges will break up quite easily.  I suggest you try an area with Sculptamold and see how you get on.  If that is not satisfactory, try some plaster bandage in (say) 25mm wide strips, dunked for a few seconds in a container, then rolled or folded into a sausage to be gently pressed into the gaps.  Rubbing over the bandage after placing, with a wet finger gives a smooth joint with the EP.

 

Harold.

[AlanRM]

Many thanks, Harold, and much appreciated. I had been wary of using PVA with EP after reports of a reluctance for it to dry in the inner parts, away from air (I have used many 2ft square panels). 

 

I have bonded several scraps of spare EP (and Surformed them similar to the pic) and will follow your excellent advice in experimenting. Using a wallpaper scraper to reach underneath the ledges is also an good idea. 

 

I'll try and report back :-)

 

 

[Oldddudders]

I know it is unhelpful, but may I say however long I look at you picture, I cannot escape the impression of waves on the seashore. Others seeking that effect might do well to study what you have done. 

[AlanRM]

This did strike me too as I was shaping it with the convex Stanley Surform shaver. It's spooky.

 

I used 10mm EP layers which build nicely towards a village, but thinner might work very well for seashores 

 

Meantime, I'm off to experiment with plaster/ Sculptamold and a wallpaper scraper! Other Half has threatened to lock me in 

[HLT 0109]

On 17/01/2020 at 08:56, AlanRM said:

I have bonded several scraps of spare EP (and Surformed them similar to the pic) and will follow your excellent advice in experimenting. Using a wallpaper scraper to reach underneath the ledges is also an good idea. 

 

I wasn't so much suggesting reaching under the ledges with the scraper as scraping upwards across the ledges so that it left a bead of filler under the edge.  If the scraper has a strip of filler along its edge and you then lay the blade agains the slope of the EP, the strip of fill will end up under the ledge.  A little practice should soon demonstrate what I mean

[AlanRM]

Thanks and yes, got this, thanks - have been experimenting!