Spray the developer on with light coats until the part is covered and looks solid white. Wait 15 minutes. If there is a large crack you should see it as soon as you spray even a little developer.
Here is the part where the suspected crack was, as you can see there is no crack! If there was a crack present, you would have seen a red line develop on the white surface. Big cracks appear almost instantly. Let the developer sit for a few minutes just to make sure there are no small cracks.
As you can see the developer detected the weld porosity just fine! We think the frame is ok for further use.
Magnaflux spray dye penetrant testing is not a substitute for wet magnetic particle (ferrous metals) or immersion tank (nonferrous) Magnaflux testing but it is a fairly accurate quick sanity check method of surface crack detection that works for both ferrous and nonferrous metals that can be quickly done in the shop or trackside by anyone and it’s a great tool to have at your disposal.