Monday, January 26, 2015

DBR1: the importance of removing mold release agents

After having problems getting primer to stick to the body, I decided that my dish soap cleaning may not have been aggressive enough to remove all the mold release agents, so I followed instructions online here, an excellent link if you can put up with the fact that the text IS ALL IN CAPS. (I've inserted the relevant text below, converted to lowercase, for the CAPS-impaired).

So I put the remaining resin bits in my strainer and soaked them in isopropyl alcohol (rubbing alcohol) for a few minutes. I followed that with a scrubbing with a toothbrush, then a careful washing in warm water with dish soap and more scrubbing. As predicted online, my strainer came up with big strings of brown goop, so something was happening.


Some tips:
  • Wear nylon (hospital-style) gloves when mucking about in the alcohol.
  • Use a fine mesh strainer sitting inside a larger pie tin. Make sure the pie tin doesn't have a pin hole in the bottom by testing with water before putting in a load of alcohol. Don't ask me how I know this, let's just say it's the voice of experience.
  • Put the plug in the sink, you don't want to lose some critical little bit down the drain hole. Do the bits in small batches so you can quickly spot a missing bit, especially the smaller stuff (i.e. do all four disk brakes together, etc.), and immediately rummage around in the sink to find anything missing.
Next: I'll have to strip the failed primer off the body parts (body, hood, doors, trunk lid) so I can give them the alcohol treatment. First, though, I'll need to get more alcohol, and more paint thinner to strip off the primer first. I'm low on acetone, too, so a trip to the pharmacy and the car parts store is in order. It's all a bit of a mess to be frank, but the optimists will be calling it a 'learning experience'. Whoopee.

Text from http://www.resinparts.com/workingwithresin.html follows, to save you all dealing with the caps; note comments on primer at the end:

"How to remove mold release from your parts:  
Assemble materials needed:
  • You will need a bath towel, cotton balls, a bottle of isopropyl rubbing alcohol, dish soap, a roll of bounty paper towels, and a toothbrush.
  • The first step is to group all parts on the bath towel next to your kitchen sink and arrange your materials so they are easy to get to.
Now fill your kitchen sink with warm water and a generous amount of dish soap (do not use extremely hot water or you could warp your parts!)
  • Use one cotton ball per resin part and wet it with the rubbing alcohol
  • Using the cotton ball now soaked with rubbing alcohol to wipe down the resin part in a circular motion to break loose any mold release
  • Mold release is silicon based and needs the alcohol to break it loose from the part
  • Failure to complete this step will not remove all traces of mold release from the part
  • Do each part one at a time and use a new cotton ball for each part
  • While the part is still wet with rubbing alcohol, place it into the sink and proceed to wash it like you would your dinner dishes using the toothbrush.
  • I recommend getting it soapy from the bubbles on top of the water surface and brushing all areas of the part before placing it into the sink to soak before you rinse
  • The rubbing alcohol breaks the mold release loose from the part and the dish soap washes it away from the part
You must complete both steps to successfully remove all traces of it from your parts.
  • Once finished soaking the part, remove it from the sink and dry with a new paper towel
Your part is now ready to work with. Use a new cotton ball and paper towel for each part you are cleaning as traces of removed mold release may be cross contaminated from part to part.
 
A note about using primer on resin parts: When it comes to the choice of primers used on resin parts, I strongly recommend that you use a sandable automotive primer. I personally use Rustoleum sandable automotive primer on all my castings and it has great results. I discourage the use of "plastic primers" on cast resin parts as I have personally had less than desirable results. In general, hobby primers lack the desired solvent strength to stick well to resin parts. Another benefit to using the sandable primer is that it goes on thicker than regular primer helping to fill any pinholes or scratches."

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