Tamiya Thin Cement linked to “Sinking Seams�???

[Yarm6]

So, once again I have noticed a few “sunken seams†on some components after cementing, puttying and painting. Background: I was gluing the engine of a 1/32 Tamiya Viper with Tamiya extra thin cement. Let it dry/cure for a day or two, then applied some Mr. Surfacer 500, let that dry overnight and sanded it down. Then I painted the engine BMF (Tamiya AS sprays). Everything seemed fine, but about 2 weeks later I noticed a small amount of “sunken seamsâ€.

I searched the forums and saw some results on using super glue, or White out for seam fixing. I’m not too keen on the super glue, but could get it to work. Someone mentioned brushing lightly with a Micro brush and then accelerator and sanding. One user posed something about Tamiya extra thin cement and the sinking seams, and when he switched to a different brand, the problem went away. It seams it’s more of the plastic moving than shrinking putty? Anyone else notice this?

Does anyone have extra information on Tamiya Extra Thin and sinking seams on round half parts (drop tanks, engines, fuselages, etc)? I have Ambroid and a Touch N Flow, should I use that on these main “seamed†parts and Extra Thin for everything else? I have even let the cement and plastic cure for a few days, then putty (happens with Tamiya putty too) cure for a few days then painting.

The sinking seams aren’t really too bad. Just annoying. Maybe it’s because it’s a bare metal finish as well.

Any help or extra insight would be more than helpful. Thanks in advance.

Dave

:D

[MarkusN]

Well, it does make some sense. Liquid cements are solvents. What these do is melt the plastic, so it can be pressure welded. The solvent then evaporates. As for the solvent that is trapped inside the seam, that can't just evaporate. It partly diffuses away into the solid plastic. As it takes up space there it makes the plastic matrix swell to some degree.

If you sand the seam in that state, you sand swelled plastic. When the solvent then completely evaporates, the seam shows some shrinkage. This basically is the same that you see on solvent based putties or tube glues that are part solids and part solvent.

Solution: Have it sit even longer to completely dry out.

That is one possible scenario. The other is that there were some mechanical tensions on the seam and the bond wasn't all that strong. The mating parts can then move away from each other, pulling on the seam and making it thinner. Nothing much can be done about this but avoiding mechanical stresses on joints.

Yet another possible scenario is the solvent content under the finish affecting its appearance, so that the seam shrinkage really isn't but just an optical effect. I have fought with that phenomenon when trying to sand seams on silver colored plastic even. You just could not separte optical effects from true geometry.

Edited August 10, 2007 by MarkusN

[dsmith]

Yes, I have this same problem too, with just about every plastic cement I have tried (Testors, Tamyia, Tenax, etc..). This is why I work on about twenty or more projects at once. I have found that after you glue something like a fuselage, wing, drop tank, I have to let it sit for about a month. Even then, the seam still shrinks sometimes. It ticks me off terribly, and is why I can't ever seem to finish a model in a decent amount of time. This is why I can never understand how some people build kits so fast....surely there are others with this problem!

-Doug

[smithery]

I found the easiest way to avoid this is to move the parts around after I've applied glue, and force some extra plastic to ooze up and leave a raised edge. I let it dry and cure for 24hrs before touching it. I can then go in, knock off the dried lip of plastic, and sand to a nice smooth finish. Any shrinkage takes place in the raised lip, and not my seam.

Since I started doing this, I have not had one single "sunken" seam.

HTH

[Yarm6]

Hey Guys-

Thanks for the insights and comments.

Well, it does make some sense. Liquid cements are solvents. What these do is melt the plastic, so it can be pressure welded. the solvent then evaporates. As for the solvent that is trapped inside the seam, that can't just evaporate. It partly diffuses away into the solic plastic. As it takes up space there it makes the plastic matrix swell to some degree.

If you sand the seam in that state, you sand swelled plastic. When the solvent then completely evaporates, the seam shows some shrinkage.

This makes sense. I checked the seam and couldn't really feel it, so it might be the "optical effect" you were talking about, but I'm pretty sure it's probably something I did. Oh well.

This is why I work on about twenty or more projects at once. I have found that after you glue something like a fuselage, wing, drop tank, I have to let it sit for about a month.

Is this a little extreme? Maybe more than 1 night, what about 2-3 days? When they mold these things, the plastic is molten, and they don’t let them cool down long before popping them out. I can’t really see why we'd have to wait a month unless it is the liquid solvent trapped in side.

I found the easiest way to avoid this is to move the parts around after I've applied glue, and force some extra plastic to ooze up and leave a raised edge. I let it dry and cure for 24hrs before touching it. I can then go in, knock off the dried lip of plastic, and sand to a nice smooth finish. Any shrinkage takes place in the raised lip, and not my seam.

HTH

Interesting Smithery. Can you elaborate on how you “move the parts around� After you line and hold the parts together, I assume you use a touch n’ flow or Tamiya Extra Think to let capillary action pull the glue in. This is what I do, and then hold them together. Do you then slightly wiggle the parts? Or push them together really hard. Sometimes I do get the “bead†but not all the times.

Thanks you’ve all been helpful! I just really want to get rid of this problem. So annoying.

Dave

[smithery]

Do you then slightly wiggle the parts?

Bingo. While moving the parts, I apply pressure as well (not much though).

This usually means I have to remove alignment pins, especially on newer kits, but I don't generally like using them anyway.

[fer_moreno]

HI! I found this post very useful...

In case I have the shrinking problem after I glued, sanded and painted both halfs of a fuselage, how can this be fixed? Do I have to sand and paint all over again or just that area? The idea is to have a smooth surface after painting...

Thanks a lot!

[billb]

i reckon that the shrinking culprit will be the Mr Surfacer. Try letting the the Mr Surfacer dry for 2-3 days (or a bit more) before sanding.

That's the big advantage of superglue as a filler, it doesn;t shrink - ever. Just make sure you sand it straight away.

All the Mr Surfacers, putties etc. shrink a great deal as they dry & keep shrinking as they cure so unless they are completely cured before you sand them they'll keep shrinking every time.

[jonbryon]

After 13 years of using Revell Contacta I got fed up of sinking seams (I call them ghost seams). I've tried all the solutions above and the only foolproof solution is to ditch poly cement altogether and make the model only using Superglue.

In my experience, Smithery's method does work, but assumes there is enough plastic to squeeze out of the joint. In practise, on most of my builds, the fit is not good enough to squeeze out excess plastic (i.e. there is a small gap, or the parts only just touch) so it's impossible to avoid the sunken seam.

That's what I think anyway

Jon

[volzj]

I found the easiest way to avoid this is to move the parts around after I've applied glue, and force some extra plastic to ooze up and leave a raised edge. I let it dry and cure for 24hrs before touching it. I can then go in, knock off the dried lip of plastic, and sand to a nice smooth finish. Any shrinkage takes place in the raised lip, and not my seam.

Since I started doing this, I have not had one single "sunken" seam.

HTH

Exactly what I do; works like a charm.

[Mist]

I started using super glue exclusively myself. I can get a bead out of the seam and then sand that bead for small seams. Bigger seams i use powdered acrylic that is used for nail repairs

[fer_moreno]

Hello guys! Thank you for the comments and tips, I will take note of them... Saludos!

[ScottD]

I was reading an article in Aerospace Modeller about Alclad last night and he stated that the problem your running into is caused by laquer based paints, like the Tamiya Spray Paint. He suggested using super glue like some other posters have suggested

I personally never ran into an issue like this and I use the bead method with my seems. If you lightly sand the parts and don't clean off the dust, you shouldn't have any problems with a bead coming out with liquid cement.

[ch9862]

Someone mentioned brushing lightly with a Micro brush and then accelerator and sanding.

Something like that is shown in Floyd Werner's DVD:

• glue plastic with any thin plastic glue
  
• apply super glue to the seam using old exacto blade
  
• apply accelerator using a microbrush
  
• sand flush
  

I had the same experience as what you're describing, I was reluctant to use super glue, but seeing this DVD made me try, and I'm a convert.

If you work on section of the seam at a time, sanding is easy - super glue doesn't have time to set rock hard. There is no chance of ghost seam showing up, since what you see on the surface is superglue. It's quick - you can go through these steps immediately right after another; I do. IMHO this approach is superior to using only super glue, because plastic glue (like Tamiya Ultra Thin) is stronger, and easier to apply.

I've heard Floyd shows the same technique in his DVD on NMF, but I don't have that one.

BTW: I'm not Floyd's cousin or business partner, just a happy customer :D.

HTH

[madmike]

I found the easiest way to avoid this is to move the parts around after I've applied glue, and force some extra plastic to ooze up and leave a raised edge. I let it dry and cure for 24hrs before touching it. I can then go in, knock off the dried lip of plastic, and sand to a nice smooth finish. Any shrinkage takes place in the raised lip, and not my seam.

Since I started doing this, I have not had one single "sunken" seam.

HTH

Yes indeed. For years the recommendation was to wait a minute after applying any liquid glue, then gently push the parts together forcing a "bead" of melted styrene up. This ensures that the join is strong and the seam can be sanded smooth without undue shrinkage.

The process of application has not fundamentally changed.

:D

MikeJ

[madmike]

i reckon that the shrinking culprit will be the Mr Surfacer. Try letting the the Mr Surfacer dry for 2-3 days (or a bit more) before sanding.

That's the big advantage of superglue as a filler, it doesn;t shrink - ever. Just make sure you sand it straight away.

All the Mr Surfacers, putties etc. shrink a great deal as they dry & keep shrinking as they cure so unless they are completely cured before you sand them they'll keep shrinking every time.

CA is a good filler for certain and excellent for parts that leave a large gap or step at the join.

I have found that Tamiya putty shrinks very little compared to most other putties.

:D

MikeJ