You can see the full tutorial by looking at previous postings on my blog. Polytek Poly 74-20 liquid polyurethane rubber is designed for making flexible, high-performance, high-strength molds. This rubber is very liquid - much more so than many of the others that I have used. Think heavy cream instead of thick syrup.
This rubber is more often used for pouring a once piece block mold or making a blanket mold. I did not opt to do either but chose to do a brush-on mold instead. Block molds are easiest for bas reliefs, as you simply build a wall about 2" all around your relief sculpture, spray with release, mix the rubber compound and pour over the sculpture. When it cures you de-mold and you are ready to start casting.
While this mold is the easiest to make, it also requires a huge amount of rubber on a high relief such as this. If the highest point on my sculpt is 2" deep I would need to pour a mold that is about 3" deep. On an 18" x 24" sculpt at 3" deep for rubber - that's a lot of wasted rubber and the costs increase dramatically. True, if you do this pour you're done - no need to make a mother mold as the rubber holds the shape well - but the cost of material can be prohibitive - especially on a smaller run.
The next choice would be a blanket mold and it would work well too - I could have done that for this sculpture. I may do this mold for the next relief.
But I chose to make a brush-on mold. I didn't have Poly Fiber, but I did have Short Stuff, which worked well enough to thicken the rubber. I can see how pouring this rubber in a block or blanket mold would terrifically easy - it is very thin and pourable and absolutely no bubbles. I have tried pouring other rubbers and most had a thicker or gummier feel and had some bubbles which can be a nightmare, making glaring imperfections in your molds and subsequent castings.
I was pretty careful, using my postal meter to accurately measure out the 2 to 1 ratio, but my gut feeling is that this product is probably a little forgiving if you didn't have access to a scale and measured by volume instead, you'd probably have it work out just fine.
The very low viscosity of the 74-20 meant that it was very easy to stir and mix well with no bubbles and it brushed on very easily with a disposable chip brush. This was a plus for me, as the rubber was thin, light with very little weight or drag on the brush so there was no moving or distortion of the softer clay sculpture. For work with hard surfaces thick rubber or heavy -handed application isn't a problem, but for softer clays, the simple act of brushing a rubber on can cause real problems.
The first two coats were applied very thinly to pick up all the detail without disturbing the clay. Subsequent layers were thickened with the Short Stuff. It did take a lot of layers to build up the thickness of the rubber on the high points, as the rubber succumbed to gravity. Perhaps Poly Fiber 2 would have made the rubber thicker than the Short Stuff that I had on hand. I felt that the time between coats was easy to negotiate - not too fast and rushed and not waiting around eons for it to set up enough for the next coat. Overall it was very simple to use - taking 4-5 hours for me to build up the mold with very, very deep undercuts to fill.
When the rubber was cured (overnight) I sprayed it with release and made the mother-mold shell in forton and reinforced with fiberglass. I could have also added leveling legs, but instead opted to rest the open mold on sandbags as I will not be casting solid, but will casting in forton and aqua resin hollow.
Overall I am very pleased with the results. The rubber was thin enough that it brushed easily without squashing my clay and I could see that there were no bubbles. The mold that I pulled off of the clay sculpture is excellent with good stretch and excellent detail - it picked up all the detail and was easy to use.