We had the exact same problem on our original car wash that we built 17 years ago (we learned and built the next two with walls that never had to be painted or stained). Anyway, we looked into every possible solution and ended up using very tall sheets of white FRP. I was sooooo hesitant to do this as every car wash I've seen with FRP looked like crap. Here's the deal, if it's installed correctly (we did the install and that makes all the difference) it looks and perfoms AWESOME. We did this remodel 12 years ago and it still looks brand new. Never has to be acid cleaned - which my other two locations do and that is a horrible job! It was by far the cheapest choice in a wall covering and I couldn't be more happy with it. We spent $60,000 extra when we built our two newest washes to have Astraglaze block walls that never need to be painted. They are gorgeous BUT they get dirty ever 6-12 months to the point that they have to be acid cleaned - and they can be damaged fairly easily. Our wash that we installed the FRP panels on is definitely in the roughest location of the three and takes a beating, yet those walls still look brillantly white, have not had a single instance of damage, and don't need to be acid cleaned. Let me know if you'd like pictures of the washes so you can see how they look. The install on the FRP is so crucial. If you decide to go with I'd be happy to share with you the tips that made our intall a success.