I'm hoping the picture shows with the irregular shellac on the inside.
Will brake cleaner and a toothbrush do it?

Probably not, but 0000 steel wool probably would. I'm assuming its mould/alignment pin lube with wax in it?

Unless its effecting how the mould drops bullets or creates fins, I'd leave it alone.

A much worse problem is getting a used mould someone who didn't know what they're doing coated with "mould prep". That junk is a real headache to remove.....

It looks like like old mold prep to me. I would try it out and if it drops nice bullets then leave it alone. I picked up a H&G gang mold that looked like this and it works beautifully as-is.

If there's build up on the flat surfaces that impedes full closure, I would scrape it off with a razor blade, then scour it a bit with oil free 0000 steel wool to open the vent lines. The cavities would probably remain untouched.

Thanks fellas.
It's a used mold I just picked up. Thought I had removed all the oil from it before I tried casting.. not sure where it was all hiding. Couldn't get a single decent drop from it

Try scrubbing the cavities with an acetone soaked Q-Tip.

Ed

There appear to be several clogged vent lines.

Will try all the above, thanks.

Heat it up by dipping in molten lead and then wipe off with a cloth rag. Nothing abrasive touches my molds...ever. I also discovered that a blast of ballistol spray lube acts like pam spray on cooking grease and makes mold clean up much easier. What you have there is hard pieces of tin oxide. They are a challenge to control while molding

Gents,
Wanted to thank you all for the very helpful suggestions. The mold cast very nicely today after brake cleaner and a light touch with OOOO steel wool. I cleared the vents with a toothpick. Was getting good drops on the second fill!

Excellent 👍

C'est bon!

Sometimes a hardwood dowel can be used to push the slag free.

Good deal. It looked like it had been sprayed with WD40.

What mold is it.

Hal

I suspect you are right.

Hal,
It's a Lyman 429421

use a pencil eraser

or brake cleaner and a toothbrush.

You can also try a bore brush in electric drill. Close the mould around it.

I bought a Hoch mold where someone had sprayed it with some kind of release stuff, who know how long ago. I tried everything, acetone, lacquer thinner, oven cleaner, nothing would touch it. I ended up using a worn bronze bore brush and Flitz. I scrubbed the vent lines clean then closed the mold over the brush with Flitz inside. But I didn't turn it because the bristles would jump over the grooves. So I just grabbed it with vice grips and went back and forth, maybe half a turn each way. Came out great! Cleaned to crap out of it and it throws a great bullet now.

personally, I'd cast with it and see how it does, looks clean enough to me.

Did you hear the mold first or clean at room temperature?

Room temp.