Probably the most important thing I've learned so far is to polish the master. This sounds simple but if you are putting modeling clay inside the car to cast the outer half, you'll probably get fingerprints on the car just by putting it down. Polish the car after touching it for any reason.

I place the car (or cars) right side up on one of the Lego Sheets leaving enough space between them to build the legos into compartments around each car. I try to make sure that the legos are at least 1/2 inch higher than the roof of the car, that will allow me 1/4 inch thick RTV over the roof and still leave me room to keep the RTV from spilling.

Getting all of the bubbles out of the RTV Silicone is very important. Most of the people that I've talked to recommend some kind of vacuum device. Now, since I don't have one of these devices (yet) I have to resort to different means. I start by stirring the mixture very slowly. This can be time consuming, but it's worth the effort. This keeps bubbles from getting into the RTV in the first place. I try not to mix more than 8 ounces (4 oz of each) at a time and I use a bowl that will give a large surface area. I use those disposable "Glad" bowls found at the supermarket. After mixing, I bang the bowl up and down on the counter, to help get the bubbles to the surface. Once at the surface, they'll break easily by just blowing on them or by stretching them with the stirrer.

Once poured, I try to allow a minimum of 5 hours to set (even though the instructions say 4 hours). In order to save RTV, sometimes I'll slice an old mold into pieces and place the pieces in the compartment making sure that they do not touch the master. I've been thinking about getting a home coffee bean grinder to grind up old molds, but I haven't tried that yet. The plan is to pour just enough virgin RTV over the car to cover it and then use a mixture of virgin RTV and ground up old molds to finish the mold. This should save on the amount of RTV I use for each mold.

Now there's choosing the right resin for the job. I know there are probably hundreds of resins available, but I've only tried 5. The 3 that MicroMark offer (CR-600, CR-300, and ColorCast) and I find that these are best left for slosh casting because of their quick gelling time. I'm still searching for the perfect resin for my two part molds, and have found the Smooth-On 327 works well and since it has a 20 minute gel time, just by jiggling the mold after it's closed gets most of the air out of the resin. Again, mixing as slowly as possible helps keep air from getting into the resin in the first place. This is easier on the slower setting resins but it's still important on the faster ones. The only way I've been able to do this is to mix only 1/2 ounce of resin at a time, enough for 2 cars (or one large vehicle).

Lastly, coloring the resin. I have in my supply cabinet, translucent resin pigments that I've picked up from the local craft store (5 colors), solid pigments from MicroMark (8 colors), and solid pigments that I've gotten from Eager Plastics (11 colors). I've also tried using Testor's Paints. All seem to work well, provided you don’t get carried away with the amount you add. Too much pigment can really distort the car. It's been not only a learning experience, but a fun one too. I now carry a t-jet and a slim line chassis with me almost everywhere and I can compare wheelbases to a die cast to see if they'll fit. I find very few will fit but when I find one, I end up with a very unique car.

MicroMark - http://www.micromark.com
Eager Plastics - http://www.eagerplastics.com
Smooth-On - http://www.smooth-on.com
Glad Containers - http://www.glad.com/containers.html
Testors Paints - http://www.testors.com