Rookie Mistake- Repairing Casts

[Xtreme TACTICS 101]

Hey everyone,



As some of you may have seen, I have finally gotten around to molding and casting my Tarkov Chest Plate. After I molded it and removed the mold from the master, I let it sit over night, then pulled the first cast in Smooth-Cast 300. After failing with my first pull (thin spots), I figured out how to fix the problem. That problem is now out of the way.



After pulling my second and third casts, I noticed a problem with the surface of my mold, and my casts. I noticed that in 2 areas of my mold the silicone (Vyta-flex 40) did not dry properly. This, I figured out was because I didn't mix Parts A and B well enough. I then looked at my cast, and in both of those 2 areas there are rather noticeable imperfections in the plastic.



I figured "Hey, I guess I will simply have to bondo they areas to fix it". Now, here is where I have hit a problem that I can't solve. I put a thin coat of bondo on the cast, and let it dry over night. The bondo was not tacky or had any problems, but the fact that it did not stick to the plastic. I thought I could fix this by roughing up the area with some light sanding (50 grit sand paper). I applied a new coat of bondo this morning, and let it dry until 4:00 this afternoon. I went to sand the bondo after I had confirmed it was dried, and again, the bondo simply slid off.



Now here is my question. Did I do something wrong? Is there a way to fix this WITHOUT making a new mold?



Thanks in advance,



Jake

Xtreme TACTICS 101.

 

[Deadalus Thorn]

I have had this happen

It can be a pain.

I used a different brand of products but same core type's, Silicon mold with 2 part Poly resin for the casts/pulls.



It turned out to be an issue with residue from the Silicon mold, although mine had mixed correctly it was releasing a moisture into the pull.

I used Easycast, procast and ultracast poly's and all had the same result, it was frustrating.



Like you, I didnt want to waste the Silicon so I queried the supplier and they gave me a few possible fixes



First was clean the mold with warm soapy water, then just prior to doing a pull, heat the mold with a heat gun or in the oven to get it warm (and I mean only warm, not boiling hot )

Do not use a release agent!!!!

Do your pull and let it cure

After demolding wash the cast/pull

Then give it a Key (Light sand)



If that does not work then try this. (be warned it is time consuming)

Have a close look at the effected area, can you see or feel (on your clean finger tips) an oil like residue?

If so, get some acetone/prepsol/pegasol on a rag and give it a good clean

then with a heat gun heat up the effected area (not to the point of warping) if you see an oil substance coming from the cast (Looks like tiny beads of sweat) then repeat the cleaning with acetone

Rinse and repeat till no more oil is coming from cast.



All going well you should then be able to Bondo and paint, although from experience some paints wont cure either



I hope this helps





Failing that you could remake the mold, but what you could do to save your loss of Silicon is to recycle the first mold you made.

You do that by grinding/Grating up the original mold, and then mixing it with your new batch (after you have done a brush coat or 3)



EDIT:

Something else that could be a factor that I just remembered.

Most 2 part resins can be susceptible to moisture, condensation can get trapped in the container after you break the seal.

I use an aerosol called a "Dry air blanket" you spray a bit in each container after use and before you store them and it helps get rid of moisture, this may be a minor factor but something worth mentioning anyway.

Moisture can interfere with the cure

 

[Cadet]

It almost seems to me that there might be some release agent still on the piece. I echo the suggestion of washing it in a bath of warm soapy water, and then letting it dry thoroughly, then try the bondo again.

 

[Xtreme TACTICS 101]

*Facepalm*!



Oh! Thats what I forgot to do. I totally forgot to wash the armour part after I pulled it to get rid of any remaining release agent! I should have known better.



Oh well, I guess concrete molding and casting is more different from armour molding and casting then I thought.



Thanks for the advice Deadalus Thron and Cadet. I'll wash the part(s) now and see what happens tomorrow night with the Bondo/Primer.



Cheers!

 

[NZTK]

You may have this sorted already but, I thouht Id throw in my 2 cents as well...

What I think is happening is this, your silicone uses an alcohol to cure, so your uncured spots are like little pools of it. alcohol inhibits urethane (SC series) there isn't really anything you can do to fix the uncured rubber except try to dig them out, you can then mix a little rubber and ty to fill the holes but your casts will always show the spots. As for bondo on urethane, it won't bond well chemically. Bondo is polyester SC is polyurethane, like attracts like. To repair the casts get some filler powder and add it to a small batch of your SC, use it like you would bondo, that way you'll have a chemical bond on the substrate.

 

[Xtreme TACTICS 101]

Ok, new problem.



I tried painting it, but the paint just chips right off. I let the cast soak in warm soapy water for 6 hours, then took it out and let it air dry. That didn't work. So, I sanded the whole surface so all of the mold releace agent was gone, and tried. The paint still did not stay on. I primed itand still no luck. I got some Krylon Fusion paint (its for bonding to plastic) and again, still no luck.



What is up. I am extremely pissed about this.

 

[Deadalus Thorn]

Ok, new problem.



I tried painting it, but the paint just chips right off. I let the cast soak in warm soapy water for 6 hours, then took it out and let it air dry. That didn't work. So, I sanded the whole surface so all of the mold releace agent was gone, and tried. The paint still did not stay on. I primed itand still no luck. I got some Krylon Fusion paint (its for bonding to plastic) and again, still no luck.



What is up. I am extremely pissed about this.

I have had this happen, almost exactly the same problem, normal primer would not key (in my case it turned out to be an old batch of resin that ended up being replaced by the supplier, but I got to keep the old batch and casts)

I worked out a few fixes, but they were all time consuming, in the end I invested in a proper Poly Resin primer which fixed the problem.



I have since found that Tamiya acrylic primer worked well, and also Citadel Chaos Black, I just bought about a dozen pots and then used a standard Airbrush and a 50/50 mix with tamiya acrylic thinner, despite what GW tell you, you don't need a special GW branded airbrush or thinner

One drawback of chaos black is it can bleed through, depending on your top coat.



Finally, you could also try an automotive Etch primer, but be aware that it has a high acid content and can cause a a pitted effect on your resin parts (depending on final hardness of the resin, I am not familiar with smoothcast so can't say what the reaction may be)

Warning: Etch primer is normally used as a treatment coat for car panels with surface rust, so only consider this as a very last resort.

 

[TheKennanator]

Something else to consider after you finish painting the piece, but before you go crazy handling it, is to put a clear sealant on it. Perhaps a spray varnish or something.



@Deadalus Thron - Citadel products FTW

 

[beaker]

take the piece and throw it in the dishwasher with dawn dishwashing soap... it *usually* will cleanup release from the surface