Here is just the little nudge you need

http://www.magnaflux.com/DocumentLibrary/OperatingInstructions/tabid/182/Default.aspx

Edit:  Liquid penetrant  "spot check"  is oil based and prolly what you are talking about.

You mentioned Tank, I guess you are referring to API 653 / 650

F.7 Diesel Oil Test

F.7.1 Diesel oil testing is required for:

a. The first pass of new shell-to-bottom welds and the final
new shell-to-bottom weld, unless the final weld is vacuum
box tested (see API Std 653, 12.1.7).

b. Floating roof deck seams and other joints required to be
liquid tight or vapor tight (see API Std 650, H.7.2 and C.4.2).

F.7.2 There are no requirements for examiner qualifications.

F.7.3 There are no procedure requirements.

I was thinking along Joey's lines. 

When I did work on petroleum storage tanks we used to come along the bottom to shell fillets and use diesel to check for leaks.  Usually on the regular and floating roofs we used vaccum boxes. 

Thanks for the info with the code numbers there Joey. 

Have a Great Day,  Brent

Floating or fixed?
Floating you spray diesel on the bottom and look on the top. On a fixed, all you can do is spray diesel on the roof angle. Only thing is if it is fixed and a frangible joint, you no want it welded too good to start with.
Whole idea of a frangible joint is for it to fail.
If it is a EFR pontoon, you want the diesel sprayed on the non welded side of the pontoon. Pontoons can be pressurized to 5 PSI and the welds soap tested instead of diesel tested. Keeps the pontoons cleaner.All the deck laps need diesel tested.API 650 Appendix C. A IFR, Internal Floating Roof falls under Appendix H

If this is a repair API 653 Appendix F if this is new construction API 650 Appendix T has the requirements for pen testing. 650 7.2.4.1d requires a 4 hour minimum dwell time for the corner weld and 4 hours minimum is considered accepted on Appendix C 4.2 ...with the methods described in this Standard.

Hint: Just before the tank is filled for hydro, spray the laps. Tanks that have passed a diesel test will sometimes leak as the pressure of the water and weight of the roof forces the diesel in the lap joints. 

Do you have a 650 and 653? You cannot inspect BRT's without them.
Go through Appendix D for response to technical inquiries. Save you a lot of time.
Also here is a link to a AST forum. http://www.ast-forum.com/index.asp
If you are in the BRT business, this is a great resource.