Wash with petrol? I've never heard that one before. I'd skip that step.
Other tips that may be useful - if you've not already tried them!
Don't use excess glue. A gentle scrape with the edge of a blade after applying glue may be useful.
Press down patch from centre to edge, e.g. with rounded end of screwdriver handle if you don't have a roller.
Before fitting to tyre, dry area with excess solvent with talc, flour or even fine dust.
And - is the glue past its best?
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