By how much does the foam expand by?

The foam supplied in DIY kits expands by about 800% over the volume of liquid chemical supplied in the tanks. That is lot of foam relative to the size of tanks supplied. The spraying process creates a myriad of tiny bubbles or closed cells within the body of the foam as it cures and this leads to the rapid rise of the foam as it is sprayed. Apply the foam in light thin layers to gauge the behaviour of the rising foam to the final depth of foam required. Cured foam can be over coated.