Cleaning between Blast and Primer

[Tzuriel]

Wow, so many questions, but as I build this thing completely in my mind before the actual start, I'll try to ask only those that really baffle me.

After the hull is completed and one starts to sandblast the interior of the hull (which is upright at this point), how do you get all that sand and dust layer out of the inside of the hull, from between the frames and plates and all those little cracks and crevices? Since after it's blasted, and the clock starts to tick before the primer laydown deadline (1 hour?), how is the interior cleaned up so much within a short amount of time? I don't even think I could do it with a shovel, shop vacuum, and a bucket of rags in half a day. Humidity here in Houston is, always high.

The pics I see make this 'white' steel incredibly clean looking, like there's no dust.

[waikikin]

Tzruriel, that job rarly happens by the "book" in real life, unless you intend prolonging the blasting pain by doing baby areas at once & coating strait away, you've gotta have help for blasting, some one to man the pot to fill it & for shutoff for safety- usually take turns about on this, then another couple for quick clean up & designate(owner/builder) some one competant to batch & mix the paint for ready application . & dont use sand, because garnet or copper slag blasting media is the go, you can often get a second hit out of it if the insides done first & the grit collected.Regards from Jeff.

[Man Overboard]

Jeff,
In reading your post, it sounds as though you are painting right after blasting. When I was a kid, I used to paint show cars. After a blast job, I would rinse with metal prep, which for the most part was nothing more than a phosphoric acid mixture. Of course you had to rinse with water, and use a neutralizer. (Lye works) You would be amazed at how much rust would leach out of clean fresh looking metal. I take it there is now a process that eliminates this step?

[waikikin]

Tom, I've only ever seen 2 wet abrasive blasting jobs on steel, 1 my dad did in the backyard on a 36' yacht, & another over 15 years ago at a swanky yacht club, the reason for wet was for dust suppression in consideration of neihbours, dont remember what dad added to the water but on the other we broomed a wet blasting additive over the steel that dried off to bright clean steel & ready for paint thats still on the boat now so must have been ok, may have been a Devoe product but its long ago.Regards from Jeff.

[Wynand N]

I usually blasted about 10 m sq at a go, then we vacuum cleaned the used grit using an industrial vacuum plant. Then epoxy prime coated the blasted area and started blasting again and repeat the cycle....

[M&M Ovenden]

Hi Tzuriel,

On our last boat we had to sandblast off old coal tar epoxy & mill scale on the inside of the hull. It was a terrible experience. We ended up blasting smaller sections and then coating them with a thin coat of red oxide to hold them before doing the final blast. The red oxide blasts off very quickly, and made the “time to paint” much shorter. Mill scale is tenacious and takes a very long time to clean off. I swore I would never do it again..... which of course I am. We are currently building a new boat and have preblasted all the inside surfaces and coated with a light primer before assembly. My advice is to look for preblasted steel, but I would also check the quality of the blast to make sure it's not just a brush blast, (as the majority of mill scale will remain).


Hi Wynand,

I can't help but think I would end up with a sandy, gooey mess taking your approach. How did you apply the paint ? How did it not get covered in sand ?

Thanks,
Mark Ovenden

[mastcolin]

There is no shortcut to industrial vacuum cleaners and air hose with blow gun.

1hour overcoat window? What primer says this? Sure you want it quick as possible but same day ie within 12-16hours is ok so long as it's kept dry.

Don't worry about absolute humidity (unless it is above 90%). More important is dewpoint ie if temperature falls, moisture will crash out onto surface. It the temps stays high/constant or rises even then humidity shouldn't be insurmountable problem.

Wash the steel before you blast ideally with water to removes salts then solvent to remove grease. Don't NOT wash with rags after blasting - you get fibres ripped off cloth onto sharp points that you won't see till you paint it.

Try to get a primer with a bit of surface tolerance. Coal tar epoxies were great for this but are pretty much banned now in most countries.

You spraying or brushing primer? Make sure you have plenty of extraction when painting, whatever. Solvent vapors are denser than air, they sink into bilges stopping ful cure of paint. ie don't just extract, arrange blowers as well.

As said, the best method is round up as many helpers as available. And I agree with comment on grit. Good grit last longer and cuts better so it will be quicker in long run.

pps sorry, SA2.5 is dust free, and yep, we get it pretty much. Dirty bits you can just disk out later. Only from opinion, SA2 is probably better than dusty (if the dust was only the grit, it would be no problem, it's the millscale dust that will rust in the paint/micro layer under paint)