Painted wiper blank issues

[M.C..]

I decided to remove my rear wiper today and instead of just using a grommet I decided to smooth it out.
steps taken
I sanded the lump off
Blocked the hole off with some plastic and bonded it in to the back face
Used P38 body filler to fill the hole
sanded smooth
Plastic Primed
Filler primed
3 coats of gloss black
3coats of clear coat
wet sanded and polished

Now I am pleased with the way it looks, it has a few inperfections but the main one is a crack around two parts of the filler, why has this happened and what would be the best way of sorting it out.
I am not a painter so it was just do what I think.

The pictures make it look worse than it is

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[M.C..]

 

[MarkX]

This happened with mine, thankfully I noticed it when it was in primer. The blank is so soft and flexible it's difficult to stop it cracking. I put another skim of filler over it and it hasn't done it again....so far!

 

[M.C..]

I did not notice mine until I was ready to put the clear coat on

 

[BGEarles]

Wiper blanks look so much better