I've got a sterling AR-180 that I took to the range after more than one year, possibly 2 years of safe storage.  After shooting, I noticed a harlin crack forming accross the top mid-section of the hand guards.  

How does the Retro experts suggest repairing or at least slowing the crack to prevent it getting worse without having to re-paint?

These are semi-gloss fiberglass resin of some sort -  I figured fiberglass patch from behind - I don't want to build up anything on the outside and repaint them - I'd rather have a hairline show than repaint the current finish.

you can use a small drill bit and drill at hole where the crack ends, that will keep it from spreading in theory.  maybe use a black rtv silicone to fill the small hole after that, should have enough elasticity to allow it to cover and the crack to not spread.

Pics would help and I would probably try superglue. I can't say I have fixed any cracks in 180 Hand Guards before. I swap them out and use the broken ones for my shorty projects.

I'd go with super glue also...

It would be hard to match the finish on the shiny 180 hand guards if you used fiber glass...

I have reworked AR-15/M-16 triangle hand guards that were really messed up with fiberglass cloth and resin but they got painted when finished...

The hand guards on Mr. Green here had broken teeth and were cracked they look like new now...

I suggest sending it to John Thomas at Retro Arms Works.  He has done some amazing work restoring AR furniture in much worse condition than yours.

Like stated we drilled small holes at the end of cracks to stop the from spreading at the body shop. I would then use some poly resin, dye it black to fill the hole and sanded crack. Then sand and refinish the area if you can find a match with the paint.

Or ...

John Thomas