Result 
How did you get on with this, has it stayed unblotchy since you did it?
My boot lid is still doing it & it is driving me nuts. Thinking back it was done in a cheapo rfu acrylic clear coat.
I am planning on respraying my rear wing this weekend so may go over it with 2k lacquer whilst I am at it.
Do you live in a hard water area? I never had this sort of thing happen when I lived in a soft water area, but now in a hard one I’ve experienced it.
Looking online they blame the limescale. It’s very frustrating after you’ve spent hours cleaning the car for it to look so blotchy - this is with the factory paint and lacquer on mine.
Yes, I do.
It can be difficult to remove all the lime-carrying water before it dries off anyway leaving behind a thin film of scale.
So I use rainwater from the water-but for the final rinse, no scale and both cars stay shiny for much longer.
So you can imagine my anguish when the water-but spring a leak at only a year or so old.
I sent a pic to the supplier asking the best way I might repair it.
Two days later they said they’re sending me a replacement on Monday!
Just as well, because the split is spreading. Here is a pic from just now before I drained it after last night’s heavy rain refilled it; the jet going up is from the original leak, but now there was a bigger one coming down from higher up where the hole is widening.
I’m pretty certain it’s the paint, it’s the only panel on the car that does it. It may be my fault compounding the paint before it had gone off properly because I wanted to get the boot lid back on.
Paint is about a year old now & the colour is spot on I reckon I should be ok 2k clear coat over it when the blotches have dried fully.
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That water-but is just your MX-5’s tin worms having a holiday and a change of food 
Nice they’re sending a replacement, I was expecting you to say they told you to buy some duct tape 
I did notice that my other car which isn’t protected from the elements (and thus gets rained on) didn’t have the problem, but it was resprayed around 2008 so I’d wondered if it were just a newer paint not being affected so much.
A week after cleaning my MX-5 the pollen managed to attach itself to the point it looked like it had never been washed, that said the limescale wasn’t so noticable now that the blue car was covered in yellow pollen.
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Hi guys. I’ve had no recurrence of the issue since I used the T-cut and wax. Though to be honest I’m afraid The Green Goddess hasn’t been out much as I’m in Cumbria and the weather has returned to type up here!
Had this exact problem many many years ago on the bonnet of a VW Beetle. Tried all manner of things but each would only last a few weeks or a rain shower.
Turned out that it was a sub-standard respraying. Cured permanently by the bonnet being resprayed.