I only do the lead test prior to renovation. Once the lead test is completed and the results show their is no lead hazard there is no need to re-test after the project is complete. If there's no lead hazard when you start there won't be any when you finish, granted that you're not re-using materials from old houses or other projects.
The lead tests that my lead inspector performs is not with a swab or dust test, he uses the XRF method (X-Ray Flourescence Spectrometer) which can read through multiple layers of paint without disturbing the surface. It looks similar to those scanners you see at a supermarket checkout except it's much larger and costs about $10,000. Even if there were 10+ layers of paint on top of a lead based paint the XRF gun would be able to determine if that were the case.
Quoted from the inspection:
"The results of this inspection indicate that no lead in amounts greater than or
equal to 1.0 mg/cm² in paint was found on any building components, using the
inspection protocol in Chapter 7 of the HUD Guidelines for the Evaluation and Control
of Lead-Based Paint Hazards in Housing (1997). Therefore, these dwellings qualify for
the exemption in 24 CFR part 35 and 40 CFR part 745 for target housing being leased
that is free of lead-based paint, as defined in the rule."