Sash channels are not that hard to repair with POR-15, or boat grade fiberglass. Here is an old post on how I repaired mine:
How to remove/install a sash onto glassBoth methods require you to paint the product. Fiberglass is much more runny so paint the sides when the surface is laying flat that you want to paint. I usually try to build up about 1/16" of whatever product I use on the INSIDE bottom of the sash. I also carefully sandblast the parts to provide a stable surface to work with. Both products will run through holes in the sash so use painters tape to seal them. I also coat ALL sides of each piece to completely encapsulate the part. I have had no problem installing the glass and cracking off the paint or fiberglass in the process.
If I have confused you post more questions.
Best Wishes, Mike