repairing burned PCBs on miller welder?

[coppercone2]

So my miller welder has got a little bit of life back in it with front panel controls being unlocked but there is still two little transistors that keep blowing up, one of them is near a patch of burn damage that I thought was OK but maybe since its some inverter control stuff, its enough of a problem that its doing something I can't measure with a DMM on the repair bench, so I decided to mill & glue the area. There is also a out of spec resistor on another PCB that I need to deal with. After I do the repair I will replace like 20 little transistors (8x bjt and 12x nmos), that possibly are damaged. Well, the whole carbonization mechanical PCB repair might be unncecessary, because the fact is the PCB was damaged, I repaired it, then the IGBT blew up and re-damaged the board, and I only replaced 2 obviously damaged parts on the board that came up as short with DMM check, so its entirely possible that there are adjacent damaged components causing the fault, not the carbon, but to preserve my sanity I decided to do a full repair on the PCB after I remove the suspicious parts this time prior to putting on new parts because its alot of work to desolder 4 terminal DIP transistors!

Plan
My plan was to use a small endmill (1/8 ball) to get rid of all the remaining carbonization (I got what I could off the top, then to pour in DP270 potting compound into the hole, possibly with tape on the bottom if there is a hole.

I am wondering if I should add fiberglass dust to the epoxy? I have a container of it, I believe its for stiffening parts, and I never filled a strait up hole in a PCB before. It also needs conformal coating afterwards, so the patch should be well compatible with the conformal coating, I thought fiberglass addition to the epoxy would help with the adhesion of the conformal coat and stiffen it further. Afterwards I would polish with a unionized abrasive (rubber abrasive).

****I also thought maybe to get glass spheres or something to strengthen the epoxy, instead of just fiberglass dust.. I see these types of fillers for sale, not sure what the best one is.

I also need a way to clean the old conformal coating off the PCB, I used a pace 'hot scraper' attachment for a pulse heater to get rid of it prior to the first rework, but I ended up replacing many parts, so afterwards my flux cleaner (flux off) ended up destroying the rest of the conformal coating so its really shitty now. I would just use more flux-off but its like 25$ a can and it is really wasteful to remove the delapitated damaged conformal coating. I figured name brand flux remover would be conformal coating safe

Then, if it works, and the thermal camera does not show any more fires, I will test it with actual welding then apply conformal coat to the reworked functional PCB, probobly from MG chemicals spray.


I also noticed another patch on the PCB where there is a dark mark, but it looks like only the conformal coating has darkened or turned amber.. does anyone know if this is a problem?