payaam2000,
I don't think that anybody can help you there. The 100% area protection does not equate to 100% cathodic protection. You still need the painting of the underside of bottom plates. Not even the paint will stop the corrosion, it will only slow down the process.
Also, it seems the Client knows a thing or two, you might learn from them.
Cheers,
gr2vessels

Sorry, and I also appologise to the welders [fumes], but I always spec bottom paint on carbons teel floor plates.  Keeps the cathodic protection from having to work very much the first decade or two.  An extra 10 to 20 years of tank life before repairs will be needed makes brush-blast and asphaltic coating look very cheap.

First of all, is the tank shop fabricated or field erected. If shop fabricated, a properly applied Coal Tar Epoxy is ample without additional cathodic protection. Just be careful when the tank is installed that the surface is not damaged.

If field erected, it is not practical to coat the bottom side without creating voids, thus also creating crevice corrosion. I thought all of the "old school" that specified this finish had either retired or passed away by now. Certainly the old timers that attempted this feat (blast & coat)aren't around any more. They either died of fright or quit while the quitting was still good!
If you can come up with a way of coating the underside of bottoms on field erected tanks while guaranteeing a 100% holiday-free finish you will become very wealthy.
Somebody will suggest that you just jack up the bottom and while supported from underneath (or by overhead crane) you simply blast, coat and once the coating is cured, move the supports & blast & coat where the supports were originally located (if you can find someone to do it!). What they won't tell you is how you repair the coating once you lower the bottom onto the tank pad and weld the 1st shell course in place. How do you repair the coating where the welding has been performed?
Most modern jurisdictions that I'm aware of require ICCP (Impressed Current Cathodic Priotection) on field erected tanks.

I guess when I read the question I assumed that they just painted the center of each plate, leaving a 2" or 3" unpainted strip around the outside edge of each plate for welding.  So then when the tank was done you would still have paint on 90% of the tank bottom.

Regards
StoneCold

payaaaaaam,
Are you still there?? So, as per the above comments, you need to pre-paint the underside of bottom plates regardless. The paint will be most likely damaged by welding, but the rest will offer some extended protection. That's the reason you have to include 100% Cathodic Protection also.

Dear GR2Vessels

thank you for your kind consideration. Finally we decided to paint the underside bottom plates including 50% cathodic protection.

Best Regards

Guys I found the above discussion very informative and have a similar query.

Normally we are painting underneath surfaces of tank bottoms in our workshop and then laying the painted plates on the tank foundations.

Welding damages the paint and also the fumes of the burning paint are detrimental to the welding quality.

To avoid this, approximately 50mm band along the plate edge areas are taped and left unpainted on both sides. (Top side becomes part of the lap joint and Bottom side comes in direct contact with the soil side.)

A client recently raised a query that the unpainted plate area in direct contact with the soil is unprotected.

Could anyone advise which is the better practice?
Painting the edges and then burning the paint afterwards? OR
Leaving the bottom underneath unpainted? and why?

Note: CP is in place.

I lean twards the 50mm / 2-inch unpainted edges.  It will leave a little of the floor unpainted that welding through paint.  Welding through paint requires a paint-specific PQR & WPS, and as you noted you have welders working in noxious fumes.

A moderate ammount of cathodic protection will take care of the unpainted strips, for decades.

The purpose of painting the bottom side of above grade storage tanks is to reduce the electrical load on the cathodic protection system that prevents soil side corrosion. It is assumed that the coating will be scratched, nicked, scraped, and burned at the welded connections but even if only 50% of the effective soil side area is still coated at the completion of construction it will greatly improve the effectiveness of the cathodic protection system.