Any tips or device on removing the moulding marks from the synthetic stock of my Mini 14?
It annoys me these rifles leave the factory like this. I’m planning on going over this with some 600 grit wet/dry unless someone has a better idea.
That would annoy me too, fortunately I've never had to repair a synthetic stock. I would see if they'll send you another and if they don't want that one back maybe try a repair?
I am not recommending this, because results that make me happy might just piss off somebody else - but years ago, I bought a used CAV15 MKII polymer AR15 lower. A previous owner had tried to remove the safety/selector (which can be difficult in some cases), got frustrated, and tried prying it out with a screwdriver or something similar. End result, he marred the lower near the selector.
IIRC, I leveled out the gouges with sandpaper and then rubbed down the entire lower with 0000 steel wool to make the entire finish match. That worked because the manufacturer used colored polymer (rather than spray painted plastic). The CAV15 didn't have any fake checkering; if it did, I would have masked it off with duct tape, like I did the serial number and other identifying info. I was happy with the result, but obviously YMMV!
I just take a sharp knife or utility knife and hold it to about a 90* angle (up and down) to the mold mark and gently scrape, going the same direction as the mold mark. Don't try to take it all off at once but make a bunch of passes. Don't try to cut it off. You can control it well enough so all you touch is the mold mark. I have never needed sandpaper. And then have fun with a few cans of Camo paint.
Synthetic stocks aren't for looking at, they're for utility. Cheap, light, strong, stable, and weatherproof.
If the molding lines are abrasive or uncomfortable, just sand or scrape them off.
You're wasting your time starting with #600. I'd actually start by working the molding line down flush with fine-cut FILES, and then switch to #320 to take the file marks out.
Getting the fixed area to visually match the rest of the stock might be difficult. Is that important to you? If so, I have some ideas. If not, I won't waste my time.