How to spackle a crack or a hole Use spackling paste that is water based. Open up the can and take out a little bit of spackle using a 2 inch knife. Make sure that you put the lid back on the can so that the spackle does not dry out. Put the spackle onto a 5 inch knife. The 5 inch knife will serve as a pallet for the spackle. Take a little bit of the spackle off of the 5 inch knife with your 2 inch knife. Make sure the hole or crack is completely cleaned out ahead of time. Take the knife and spread the spackle over the hole or crack leaving a little excess on the surface. Then angle your blade more vertically to the surface and scrap off the excess spackle leaving only the crack or hole filled with spackle.