Realise the importance of selecting the correct techniques. Know the advantages and disadvantages of the different penetrant methods, developers etc.
Know when it is prudent to perform bleed-control and bleed-back actions.
Clean lint-free rags are exactly what they are. If you use sub-standard rags, you will soon realise that they will hinder you in performing a good inspection. Remaining lint on the inspection surface can be a real pain especially when looking for tiny indications.
Prevent your inspection area from becoming too cluttered; you could increase the chance of missing a crucial indication.
If your initial bleed out is profuse, try to size the indication (if the code allows) before final interpretation/evaluation in order to acquire a more accurate measurement.
