First you close the nuts and holes with wax dripping from a candle than sand the firewall so it will be even,
Than you mix 30 min epoxy and spread it all over the engine compartment with epoxy brush and a heat gun again so it will be even,
At this stage its sealed.
then you can paint it with midwest aerogloss or some other paint that stands well the oil and fuel.
can do without the heat gun but its more beautiful with, I also see that at tower the aerogloss paints are out of stock....[]
can also thin the epoxy with alcohol but I didnt try that.