Your dad is mixing the mixture that can be used to fix the hull. It's pH is off before adding the final ingredient.

You could check the pH with a tool. Something that know what you were eating.

Dip you spoon into the mixture. It will tell you the pH.

Ask your dad how to lower the pH.

You know, the peach

Put some peach into the Hull Patch to lower its pH. Check the pH with the spoon.

Applying peach into it lowered the pH by two from 14 to 12. Just put some peach juice into it until you have lowered it enough.

Oh, you can't get to seven, right. First, lower the mixture to pH of 6 with the peach juice.

Now you need to raise the pH just by one. Ask your dad how.

Get some calcium carbonate

Great, put the egg shells into the hull patch. Check the pH.

You just have to apply some peach juice (-2 pH) and egg shells (+3 pH) multiple times to get it to 7 pH. Check with the spoon.