What is the best filler/cement for a large styrene model?

Is there a way to get a piece in behind the gap/ joint? Otherwise it's bound to crack back open at some point.

Maybe UV cure glue?  It is acrylic resin, and supposedly adheres well and is strong.  I have been wanting to try it out myself, but have had no luck sourcing it where I live.

BTW, what are you making?  Can you share pics, or is it Super Secret?  

GMorrison

Is there a way to get a piece in behind the gap/ joint? Otherwise it's bound to crack back open at some point.

No, the join needs to be like an invisible join. i.e. It needs to be the same thickness as the base material (i.e. 3 mm thick) and sanded on both sides soas to be flush.

Real G

Maybe UV cure glue?  It is acrylic resin, and supposedly adheres well and is strong.  I have been wanting to try it out myself, but have had no luck sourcing it where I live.

BTW, what are you making?  Can you share pics, or is it Super Secret?  

Can think of two approaches:

Scrape opposite sides of the opposing parts to half thickness to create a half-lap joint 

Bevel the joining surfaces, one or both sides.  Glue a piece of styrene stock; round, half round, or square into join. I’d use Tamiya Thin, Tenax, or MEK as the glue.  Scrape/sand flush

I have found that UV glues harden sort of rubbery.  Sanding is problematical  May work for nav lights but ...

EdGrune

Scrape opposite sides of the opposing parts to half thickness to create a half-lap joint 

Sounds intriging. How would you actually do that? Is there some clever tool or machine that would allow me to accurately cut a strip of styrene to a fixed width and depth.  Part of my original problem was that I did the original cutting on a bandsaw which then created a wobble of about 1mm in its cut. I have been trying to 'fettle' it with selective filing by hand into a better fit, but it's extremely time-consuming. I also find sanding by hand has a nasty habit of accidentlly removing more material at each the end the strip being sanded...

EdGrune

Bevel the joining surfaces, one or both sides.  Glue a piece of styrene stock; round, half round, or square into join. I’d use Tamiya Thin, Tenax, or MEK as the glue.  Scrape/sand flush

You are trying to do something that's next to impossible to do. Imagine joining two sheets of plywood together edge-to-edge.

Glue bonds things together much better in shear than it does in tension. That's Ed's first suggestion.

In the real world, you would weld the two pieces together if they were metal, and grind the bead. Ed's second suggestion is in that vein.

Maybe if the two pieces are flat, they could be keyed together. That might take the form of a piece of styrene the same thickness, an inch or so long and fairly narrow in width; being glued into a slot in each half.

GMorrison

You are trying to do something that's next to impossible to do. Imagine joining two sheets of plywood together edge-to-edge.

Glue bonds things together much better in shear than it does in tension. That's Ed's first suggestion.

In the real world, you would weld the two pieces together if they were metal, and grind the bead. Ed's second suggestion is in that vein.

Maybe if the two pieces are flat, they could be keyed together. That might take the form of a piece of styrene the same thickness, an inch or so long and fairly narrow in width; being glued into a slot in each hal

 

ship69

 EdGrune

Scrape opposite sides of the opposing parts to half thickness to create a half-lap joint 

 

 

 

Sounds intriging. How would you actually do that? Is there some clever tool or machine that would allow me to accurately cut a strip of styrene to a fixed width and depth.  Part of my original problem was that I did the original cutting on a bandsaw which then created a wobble of about 1mm in its cut. I have been trying to 'fettle' it with selective filing by hand into a better fit, but it's extremely time-consuming. I also find sanding by hand has a nasty habit of accidentlly removing more material at each the end the strip being sanded...

 

 

 

EdGrune

Bevel the joining surfaces, one or both sides.  Glue a piece of styrene stock; round, half round, or square into join. I’d use Tamiya Thin, Tenax, or MEK as the glue.  Scrape/sand flush

 

 

I'm not sure I really understand what you mean. Yes, I guess cutting a bevel at say 45 degrees would increase the surface area of the join compared to a 90 degrees edge. However I can't see how either round or half-round styrene stock would help create a strong, seamless bond.

Morrison understands

I would build a scraping jig.  The end stopper would be a piece of metal (brass, steel) which is ideally half the thickness of the part, here 1.5 mm.  It should be fairly wide 2.5 cm (1-inch) or so.  You will also need another piece of metal which will be your depth stop.  

Slide the workpiece onto the jig to the end stopper.   Clamp or secure the depth stop on the top of the workpiece at your desired halflap depth.  

Hold a single blade razor blade or scraper vertically on the end stop and flush against the depth stop.  It will be on an angle with the workpiece material to be removed. Drag the across the workpiece removing material until the cut bottoms out.

Repeat using the same setup on the other piece, remembering to flip sides.

Glue and clamp

As far as the weld bead bevel,  doing some back of the envelope cipherin' using some of that math stuff you're supposed to remember from school -- the Pythagorean Theorum.  If you bevel at 45 degrees the edge such that the bevel is 3mm deep and extends back 3mm from the edge,  the gluing surface which you are going to fill with a piece of stock (ideally 3mm square stock) will be increased to 4.25mm.   This is over the former 3mm flat gluing surface.

hypotenuse == sqrt(side1**2 + side2**2)

For larger cracks I use auto body filler, usually called glazing compoud or glazing putty.  A big tube is expensive, but you get a lot- it is a big tube!

For very small cracks I use paint.  I do not stir the bottle- I dip a toothpick into the thick stuff at the bottom of the jar and apply it with that toothpick.  Sometimes it takes a couple of applications.  I use Testors enamels, and always use flats when I use it as filler, so it dries faster.

I think you've got what you need.

The combined thickness of your lap joint can be slightly thicker that the sheet itself, since you plan to sand it a little. But, the closer you are at assembly, the less work later.

I'd definitely work from samples for a bit before you tackle the model. Solvent will make the edges of your sheets distort, it has a habit of running outside the joint.

There's the second oldest carpenter's adage: there's never time to do it right, but somehow there's always time to do it over.

> I would build a scraping jig.

I only half-understand that. I guess you are proposing to use a square razor-blade, yes? Either way, I didn't know that you can scrape styrene like that. Are there any videos on YouTube of this sort of thing?

Either way, how do you end up with a clean, accurate edge to the area that you are scraping? And a clean, accurate edge would surely be pretty important, unless of course you are prepared to fill the gaps with something...

For now I have been filling with layers of a goo made up of off-cuts from my base styrene dissolved in MEK-PAK solvent/cement. Clearly it won't be as strong as what you are suggesting but once the MEK evapourates fully it seems to be fingernail proof which will should do me for now. And if it falls to pieces I can always try cementing it back into place!

Cheers

J

Another idea.....clamp the sheet down with a thin metal straight edge and sand from the sheet edge to the metal edge.  If using 3mm sheet, would suggest a 5 mm or so setback from the edge to the metal straightedge.  Flip the second sheet and repeat. This will give a bigger glue area than just a butt joint, and just have minor sanding/filling on the two seams.  Search "scarf joint"  for diagrams if need be.

Same idea as the half lap joint shown above, without trying to carve a flat joint half the thickness of your sheet (rabbit cut for a half lap joint).  if you have a trim router, could try to cut the rabbit with it and a sharp bit.  On some scrap first, use a straight edge to keep things square and make shallow passes, maybe a couple to get half way on a 3mm thickness.