Working with foamboard

[Dog Star]

I am building a road over rail bridge from 5mm foamboard (as sold by Hobbycraft) - this product is some form of extruded foam interior with a thick paper layer on each side of the foam. As the bridge is skew to the line of the railway the bridge features "helicoidal" arches and therein lies the root of this question. The front face and rear face of the bridge are parallel, the vertical faces of the arches are parallel - the curved, underneath, surface of each arch is (roughly) a parallelogram with the long sides as drawn-out 'S' curves.

To get the underneath layer of each arch I have made a template for use in cutting foam parts. As the shape of the piece is dependent upon the "skew" angle and the radii of the arch I have gaps of up to 2mm between the underneath piece and the front / back of the bridge.

What can I use as a filler of the gaps where the arch piece joins the front / back faces of the bridge?

thank you, Graham

 

[Steve Cook]

Silicon sealant, left to dry then a smear of any kind of filler over the top?
I'd probably use the lightweight filler available from Tesco - you almost feel like you are buying a pot of air, its easy to put into place and smooth with a finger and it sanded well without being too hard.

You could fill in the gaps with strategic applications of card strips and pva glue as another option.

A photo might help too Graham.

Steve

 

[Dog Star]

Thank you Steve.

As I can get to the reverse side of the joins then I shall use your suggestion of using Silicon sealant to "block" or "blind" the gaps. Thereafter I shall use a fine grain filler from the viewing side... a filler which is amenable to water hence fill and wipe over to avoid sanding the structure.

For a photo of the prototype... try here .

For a photo of the model without the roof of the arches... try here .

 

[InvernessTMD]

Just be careful with the foamboard soaking up water and warping...

 

[J_F_S]

Silicon sealant, left to dry then a smear of any kind of filler over the top?

Steve

I notice this suggestion ends with a question mark - have you tried it Steve? Personally, I would not use silicon sealant for any purpose except sealing because the merest trace of it will cause problems with any kind of paint - at best it causes "cissing" (see for example - cissing paint - Google Search)
At worst, neither paint nor anything else, will stick. Worst of all, this is an effect which lasts seemingly for ever - it does not end when the stuff "dries". And if you get it on your fingers, anything you touch transmits the effect like the plague.

For that reason, it is banned in our house except under military standards of control!! If you are trying this idea Graham, I suggest doing it in the garage first...

If I were doing this, I would use 3mm MDF, leaving some meat on both the spandrel walls and the arch so that you can then blend the edges by sanding. You can buy "bendy" MDF for the arch - it bends very easily. By sanding the sides of this to be vertical after bending, you should have no gaps at all, and any that do remain are small enough to fill with wood filler. For the reason given by Inverness TMD, I would avoid anything water based with foam board - it can go all shapes.

Howard

 

[3 LINK]

Hi Graham,

I agree with Howard, nothing like paint will key to the silicon sealant. It has its place in the market but not for gap filling.

Silicon actually has good grab qualities on par with adhesives like " No nails, etc " , obviously the joint will stay flexible but the grip itself is quite impressive.

If the gaps you are left with are on the small side, I would recommend Decorators Caulk which can be applied with a silicon gun, and it is manufactured to take paint once dry, it is cheap to buy and is available at most of the DIY's.

Martyn.

 

[Dog Star]

All suggestions and warnings have been accepted in the spirit of being contributions to WT-ing so no problems.

Steve's (@Steve Cook) idea of silicon sealer works for me in that I can get to the back of the join and hence the sealer "blinds" the gap thereby allowing more refined fillers to be applied from the front. As the back of the join is in an area of the structure which is not visible once the road deck has been installed then there is no requirement for painting over the silicon sealer.

Taking Simon's point (@InvernessTMD) about damp and the foamboard covering I could have sealed the surface of the material... in truth I changed direction and filled the gaps with UHU Pur (this is a non-solvent form of UHU).

Howard's (@J_F_S) suggestion of bendy MDF is eminently suitable and one I might use if (a) I had need of doing the work again and (b) the local Triumvirate of "did-it-myself" warehouses could provide the stuff.

... Decorators Caulk ... thank you Martyn (@3 LINK), I like that idea.

So the bridge is now "complete" and ready for cladding with Slater's brick plastikard for the underneath of the arches and Depron for the stonework. A field trip is needed at this point for photographs of the stonework (colours and textures) plus the bedstones from which the brick arch is sprung.

Continuation of the bridge saga shall be in our layout topic on Scruft's Junction.

 

[Steve Cook]

I notice this suggestion ends with a question mark - have you tried it Steve?

Yup, thats why I mentioned it. I recommended it on the basis that it fills the majority of the gap and remains flexible, whilst the accurate filling and sanding takes place with the filler over the top. Directors caulk as mentioned by Martyn would allow the same end result and avoids the risk of silicon if ones perceives that to be an issue within this application.
Steve

 

[Geoff]

Excuse me for butting in chaps,

I've been experimenting with foamboard myself and use Milliput to fill any gaps. Once set it can be sanded back and scribed with ease, as for the board itself, well I gave up with the Hobbycraft product and bought a large sheet of 3mm board from Trent-Plastics. They do it in several thicknesses from 2mm up to 19mm, and provide an excellent delivery service.

Matt White Foam Sheets - Trent Plastics Fabrications Ltd

This is a far superior product, being more rigid then the Hobbycraft board, it looks, and feels like a thick styrene sheet. Cuts very cleanly and takes scribed detail very well. Once I've completed a building or structure, I spray it with Halfords acrylic primer, and it will then take enamels, acrylics and even emulsion paint.

Geoff

 

[farnetti]

Evostik make a great product called 'Sticks like Sh*t'. It is a very strong adhesive, about the consistency of silicon and used in the same way but does take paint.

It is a trade only product though so you would need to go to a builders merchant or Screwfix to get it.

Ken