That reminds me - I should check my doors to see whether my touch-up did the job or has started rusting through. If I remember correctly, I used some rust treatment (the type that turns rust black) then some zinc primer and a standard VW touch-up kit. I may have first rubbed it down slightly with a glass-fibre pencil?
Anyway, I moved on to OEM style front mudflaps (keeping the moulded ones on the rear), initially pattern ones, but they rusted quickly and were quite stiff, so with occasional bumping off kerbs (done carefully - but the mudflap would still catch) they didn’t flex enough and eventually the metal support/mount strip fatigued and broke. Genuine VW ones are a fair bit more flexible.