Yes, you'd need to wash the car before hand!
Liquid Glass does have it's following but, is an old product and IDK if they have ever changed the formula. It's been around for over 30 years that I know of and probably waaaaaay longer. I only first heard of it 30 years ago!
At one time it was probably the cat's PJ's but with all of the new product today on the market that are much le$$ expen$ive, I would buy something else for L3$$.
I have used it and was not impressed in my own side-by-side comparison between something like, 5 different products on the test hood of my car(years ago). It only seemed to do the same thing as the other products that cost less money. I just didn't see the difference in gloss enhancement or longevity.
When I tried it 26 or so years ago, LG cost ~$16 compared to the other products that were between $5-$10 for the same qty.
Now, I'm not slamming LG nor I'm not saying that the LG didn't do what it said it would do, I'm just saying that I didn't see the difference between it and other less costly products.