Semi-Gloss clear polyurethane is pretty durable and seems to be proof against Hoppe's.
You can also use semi-gloss brushing lacquer. Lowe's hardware usually sell it, but it's in cans that are much larger then the little cans of Minwax clear poly that you can buy at Walmart.
If you only paid $125 for a set of genuine Colt First Type Target grips you stole them.
These days those grips START at $250 and go UP.
What I recommend is to use paint thinner to thin the clear poly fairly thin for the first coat. This allows good penetration. Also brush a light coat on the checkering. Allow to dry until really dry before applying more coats.
For following coats, just thin about 3% to improve brushing.
Apply one of these 3% thinned coats to the checkering, but don't apply more or the checkering will fill up.