Fixing voids in acquired mould

I just picked up a mold from a yard for building small racing prams. I'd say fewer than ten boats were pulled from this mould but I plan to build 50 - 100.

After cleaning it up and giving it a polish / wax I noticed about a hundred small (pin head to dime sized) bubbles / voids. I've dug each out and now I'm looking at what to fill with. In the past I've used small amount of vinylester gelcoat, Duartec vinylester putty, and polyester fairing compound for repairing a mould. The vinylester products, of course, are *very* tough to sand smooth but I assume are hard. I know I'll have to resurface the mold so maybe just fill with fairing compound?

Thoughts? Thanks, don

 

Lower cost and easier to sand fairing compounds can work, but they typically shrink after a few parts are made and become noticeable.

 

How does that happen, the shrinking after a few parts are made ?

 

Polyesters shrink, well, all resins shrink as they cure, some more than others.

The lower the cost and the easier it is to work with, typically the more it shrinks.

After you make the first parts the fairing compound tends to cure more, so the repair changes shape.

 

Would filling (overfilling) and waiting for it to fully cure, before sanding, solve that ? What time frame would that involve ?

 

In tooling applications the low cost fairing compounds tend to continue moving at a different rate than the rest of the mold. So even when you think you've got it right, it changes.

 

Since it's a small mold and you're going to pull that many hulls out of it, you might consider pulling a master plug out of the mold you have, working it to perfection and then making a new mold without all the imperfections. Then when the new mold gets beat up after making a bunch of pulls, or destroyed after a bad layup, you have the master plug to easily make another mold. Also, if you want to speed up the production of 50-100 hulls, having 2 or 3 working molds will help.