Google "flame polish"

https://www.instructables.com/id/How-to-Flame-Polish-Plastic/
Practice on some scraps first until you get the hang of it...
EDIT: that was the first link I found...may not be the best, look around and you will find some usefull info..

I'd find an old ramp somewhere before you practice a first time flame polish job on a WW. It's not as "hard" as it may seem, but it's an acquired skill, and any prior experience, even if just a little, will make all the difference in the end result.

Look on pinside for guides.

Novus 2 works. I've used it more than a few times on ramps and have had great results

Any fine automotive polishing compound will work fairly well.

Novus 2 and then Flame Polish

Ditto^^. I used a soft polishing wheel and a drill.

I've seen some rather impressive posts here with the flame polishing - never tried it tho.

Please remember, if you don't clean the ramp first you're just going to MELT the dirt into the ramp forever.

Novus 2 & headlight cleaner are all doing the same thing and Novus is cheaper. Then you can go clean your headlights with it.

I have used both a flame and a very hot heat gun I have. I like the heat gun better because I find that I have more control.
YMMV.

That's interesting to know. I'm guessing it takes a little longer than a flame and not directed as well as a flame? if so, any warping issues?

I’m curious what you use

Either the flame or the heat gun can soften the ramp. It takes practice and patience.
On my test ramp, I also got the plastic to bubble with the torch, not good.
As I said , I have used both depending on situation
The results can be dramatic

I believe in flame polishing ramps to clear up ramps. I use a standard propane torch that you can pick up and any hardware store. I use novus 2 like suggested above to clean heavy scratches. I move to novus 1 to do a fine polish then clean with soap and water for a final cleaning. With the torch i use you have to be moving fast and pay close attention to make sure you get a uniform finish.

All in all flame polishing is the way to go in my book. I would get something to practice on first to get the hang of it. Goodluck

So what do you guys do if there's decals on the underside of the plastic?

i wouldn't recommend flame polishing if you haven't ever done it. ive done many ramps, and it takes a lot of practice and is very, very easy to mess up a ramp quickly. some ramps polish great, others won't polish. for instance.... your lower to upper ramp cannot be flame polished. the ramps of whitewater are not the ramps you want to learn on. too big and heavy. anytime you heat up one end, the weight of the ramp is gonna cause it to bend where you don't want it too. stick to short ramps for learning on. very short spurts of heat and keep torch moving or it will bubble. if you take torch over a crack or split, it will make it 10x worse. clean thoroughly with dawn dishwashing liquid and remove all dirt from ramp before applying heat. can't polish where there is a decal either.

Whitewater's ramps are pretty forgiving.

Ive had pretty good luck flame posihing ramps with decals. Remember your not really getting the ramp real hot. Your heating the ramp up just enough to blend all the imperfection together and smooth out. As long as your moving quick on the opposite side of the decal you should be ok. Do not put heat on the bare decal or it will burn it. You must stay on the opposit side.

Not the whirlpool funnel. It gets real thin near the end that goes through the playfield. I let it get too hot once and it deformed slightly in a hurry. Deformed just enough the ball would get stuck. Luckily it was right at the end so I had to cut an inch off the end. But pretty much the other ramps are fairly thick.