There are a few things to note on the topic of shampoo, annnnd how far down the rabbit hole you want to go....lol
Shampoos of recent are much of a muchness in their cleaning ability's but a few stand out for me. If your after cheap dilution ratios and something that will sheet off a little easier, DoDo Juice do something called their supernatural, I've had 250ml that I use on costumers and friends cars (anywhere between 3-5 cars a week) and it's lasted over 5 months. Blown away really. I hate snowfoam as I think it's all hype, much better using a pump sprayer and something like BiltHamber Auto foam through it, better results from all my testing anyway
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Going back to shampoos, Zaino Z7 is AMAZING and if I'm just after topping with a good gloss enhancing shampoo it's my go to.
If you are after throwing down a layer of wax/sealant then you want to be looking at a few things also,
Do you want your wax/sealant being affected by the attributes that the shampoo will leave behind? For me this is a no, I havent spent 50 hours perfecting my paint work, putting a coating on and throwing down Zymol Concourse to achieve a truly amazing finish to only ruin the water behaviour and look from a £15 bottle of shampoo lol.
If you are after going down the wax route then in my opinion you need something with no additives or enhancers. Definitive wax do the VERY BEST I have found
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But then if your still washing with a bucket and sponge and putting more and more scratches and swirls then it may as well be with fairy liquid as your killing it anyway
Go check out Detailing World, it'll blow your mind hahaha. Don't come and shout at me later though....