Sir,

apart from what has been said already on your topic. There is, and no offence intended saying that, rather little technical information content in your post(s).

That is. What are the boundary conditions of your welding application? You know, it's difficult for me to understand something connected to welding or to a particular welding application, as long as the "whole story" hasn't been told. The gentlemen who did so well provide you with their advice and estimations on how to overcome your issues may have been right. However, due to lack of detailed technical information and despite their certainly purest intentions, they might even be wrong.

I myself am not the one who tries providing advice based upon vague speculations. Short. I would tend to avoid issues, i.e. the "soot", whatever that means, before being faced with the need of fixing the issues, i.e. in your case to "clearing" it afterwards from the tank interior.

However, before knowing how to avoid something, one should know the boundary conditions basically leading to its generation. As a consequence, some simple questions came to my mind.

Your original post contains as follows:

1.  Quote: "I am MIG welding (…)"

•  You use to use the term "MIG Welding". I'm asking does this refer to e.g. British Standard BS 499-1:2009? You know, it’s always hard for me, coming from beyond the pond, to not confuse the presently valid AWS terminology, say Gas Metal Arc Welding (GMAW), with the European one based upon the differentiation between MIG (Metal Inert Gas, i.e. an inert gas is used for shielding) and MAG (Metal Active Gas, i.e. it is an active gas employed). Short, I would correlate MIG to non-ferrous parent materials rather than to ferrous, say "Carbon or Mild Steel" Grades. Let’s hypothesise the tank is made of an aluminium alloy parent material, which of course can be joined using MIG welding. Then the "soot" (in my understanding this strange "dark matter" often connected to using the MIG process for aluminium alloys containing particularly magnesium) might understandably arise from the critical combination of process (MIG) and material (parent- and/or filler metal). Then, in order to avoid that, I would tend to ask: "Why not switching the welding process from MIG to TIG (GTAW) using alternating current?" That by the way would also beneficially contribute to meet the requirement (quote): "...tank should be free from (…) Spatter (…)" since TIG (GTAW) actually does rather not produce any of these.

2.  Quote: "Presently I am using HRPL A 36 material."

•  I ask your forgiveness for being unaware of what "HRPL" means. There are too many strange acronyms around nowadays and I admit to literally not understand all of these. However, I seem to interpret "A36" being some sort of steel. Maybe similar to 'ASTM A36'? If that is so, well, then at least my first question above regarding non-ferrous material MIG Welding has already been answered. However, in conjunction to this and always intending to clarify the boundary conditions that primarily may cause the "soot" to arise, I should tend to ask: "Have you ever considered to "purge" the tank after adequately cleaning the parts prior to welding, e.g. using argon?"

I may be wrong, but I suppose that at least might relieve you from ending up with greater amounts of oxide layers at the inside, that might "attract" the "soot" or may drive/cause the "soot" to adhere to. In combination with adequately adjusting the welding conditions, i.e. the process parameters used (e.g. reduce "heat input" for the first pass(es)) that might at least improve somewhat your current situation.

But of course and besides of all I said; as Professor Crisi from Brazil correctly was stating: "Now, if by "soot" you mean another type of material, I'd like to know what it is."

I tend to agree. It would be interesting to know what the "soot", in your understanding, actually is.

In short. As long as we are not becoming aware of some further details that make us understanding the surroundings of your particular application, as long it's like taking a stab in the dark or - simply hard to advise.

Anyway, I'm one of those who use to say: "A stitch in time saves nine."
That is. I would tend to prevent the "soot" even to arise before being faced with the need of clearing it from the tank.

Thank you.

But, when diluted to the levels used to clean shop floors, swimming pools (they sell it pre-diluted for these applications),  and prepping steel for painting, etc, you don't have the same corrosive problem.  Though, it is important to remember that all acids are corrosive, some are just slower about completing their work. Plus, you have to have access to rinsing so that you remove any residue after the cleaning.  Even the water normally used to dilute, rinse, cleanse, surfaces is slightly acidic. 

Do not breathe vapours, is very sound advice for any cleaning products. 

But, the main point here has become one more of proper prep before welding and not having to clean soot after welding.  The hydraulic fluid residue causing the soot should be able to be adequately removed before repairing the part.

Have a Great Day,  Brent