Thread: spray gun and little black bitties

Originally Posted by old_picker

currently spraying a solid colour onto a small job and am being plagued by tiny black impurities showing up in the finish - sieving the finish through a 150 mesh screen doesnt help so i am beginning to suspect it is coming from my compressor -

its a gmc 45 litre job that i bought 3 years ago - still pumping really good -
is it common to get dirt in the air?
can the thing be cleaned out?
can a filter be used?
it does have a small moisture trap that seens to work ok

When you say moisture trap do you mean built in to a regulator to just a tap at the bottom of the tank?

I've had a similar problem, but draining via the tape on the tank, and a moisture trap built into my regulator stopped most of it. Later on I added a third smaller trap at the base of my gun and havent had a problem since.

joez

i just have a small trap at the base of the gun

If you dont already , have a look at the base of the tank see if you can drain it.

If you are going to spray properly you need a better system than you have.

First as suggested drain the compressor every day either before you spray or whats better at the end of the day when you turn off the compressor open the tap on the bottom of the tank. Unfortunately laws reguarding compressors are not being looked at too carefully these days, by law there must be a tap on the base of the compressor tank, cheap compresssors often now don't have one . If not it would be an idea to dril/tap/weld one into the tank.

Compresssed air for spraying needs to be put through an air transformer.

This is a filter, moisture trap and a pressure regulator.

The filter cleans out any dirt/rust/oil or other contaminates from the compressor.

The filter is also set up to cool the air down and expel any moisture which is in the air. When you compress air you heat it up and the moisture in the air turns to steam and it can come through the hose to the gun and cool down and mix with the paint. This can cause lots of problems so it is usual to have at least 5 metres of pipe between the compressor and the transformer.

You should be able to google and find how to set this up. If you take the pipe up the wall and then slope it water will cool in the pipe and you can collect it before it gets to the Transformer with a T in the pipe.

The drier the air at the transformer the easier and longer it is before the transformer need to be drained off.

Don't forget other possibilities such as the hose. often old hoses break down and the air takes the material through the hose and into the air to the gun.

Strain the paint with a lot finer cloth than you have. use a nylon stocking or get some silk screen material. You can also get disposable paper filters and special filters to put in the paint pot to stop foreigh bits getting to the surface being painter.

If the compressor is putting out lots of muck as you suspect you can add filters to the line before the transformer. If you intend doing a lot of spraying and find the compressor is causing this problem it may be better to invest in a better quality one

All very good advice

Last but not least, don't forget the spray gun. If this hasn't been cleaned properly last time it was used, ie taken apart and properly cleaned, paint will have dried in it. This dried paint can be lifted by subsequent paint put through the gun, particularly if the last paint used was an enamel and this time you are putting through a lacquer. These lifted bits of dried paint then finish up in your wet paint film.

thanks for all that
btw i am sieving my lacquer with a 150 mesh svcreen which is way finer than a stocking - in fact i have a devil of a job getting it through 150 mesh - even 100 mesh is slow

the tank does indeed have bottom tap wich i used to drain the muck out of it -
i installed a new reg/filter/water trap at the compressor end - the old unit was just a reg -
also replaced the moisture trap which is attached to the gun - this was full of of black crap which looked suspiciously like what was blowing into the finish

havent fired it up as yet but am confident that it will blow clean now

Originally Posted by durwood

. If not it would be an idea to dril/tap/weld one into the tank.

An air reciever tank is a pressure vessel. I would be very careful about attempting anything like this, unless it's done by a suitably qualified person, and inspected/tested as per the relevant standards.

I am not trying to say I'm an expert on this, but I would hate to see someone get hit by a tap and fittings when the air pressure built up

I did a bit of a search on the internet on air reciever tank regulations,
it seems that unless the tank is used in industry etc. there may not be any restrictions what you can do to it.

I would still be very careful about any modifications, this type of setup would have a very thin metal tank, any drilling or welding is likely to weaken the tank.

One thing I did find was this type of tank may have a "use by date" stamped on the tank, much like an LPG cylinder.

I used to have to teach the regulations in regards to compressor safety requirements.

Somehow the laws have either been changed or as is usual the ball has been dropped. I don't think reducing the requirements would have been done ( I may be wrong does anyone know?) Its more likely that those who used to be responsible have retired and its been let slip with the massive amounts being imported.

Years ago hardly anyone had a compressor at home unless they pieced it together themselves out of old fridge bits etc.the only ones sold were proper industrial approved ones which inspectors were able to keep track of, bit hard now with Bunnings and everyone else bringing container loads from overseas.

The law stated a compressor sold commercially must have a pressure gauge and a pressure relief valve on the tank, a drain cock on the bottom of the tank an inspection hole to allow interior supervision of the condition.( to check for rust ). The relief valve is sealed with a proper lead seal and is not supposed to be tampered with. You didn't have to have an automatic cut out switch but if you did you still needed a separate relief valve.

In industry the compressor should be checked yearly and a certificate issued ( last one at work was over three years old ???) All compressors sold were to conform as they could be used in industry and therefore need to be correct. If you make your own up for home use you can do whatever you like just don't employ anyone or you become an industry and the rules apply.There a lot of compressors on ebay and in stores now which don't have all or any of these features.

Poorly done a drain tap put into the bottom may be let go but it would have to be a pretty poor installation. if it did let go it would be close to the floor and would not be likely to fly around. I think no tap, rust build up inside till it was too weak would be an even worse danger.

If you have a tank and its not possible to see the condition inside then you should think about the situation. You don't know how long it will be before it bursts from weakness caused by rust. The tanks once welded up can't be painted inside properly and are usually bare metal if it has an inspection hole you can shine a torch inside to check it as well as clean and rust proof it so it lasts longer.

The original regulations were made in the 1950's after a compressor exploded in a hospital in Tasmania killing people. My boss was forced to buy a new compressor with the inspection hole etc in the tank as the shops compressor did not have an inspectuion plate. Being an industrial compressor it was a thick steel tank not like the tin type being flogged of from China. Even some of the better ($500 plus) compressors being sold now aren't right. A garage or other small business could buy in good faith buy what they think is a good compressor only to find its knocked back by the inspectors when he checks out the premises.

If you are looking for a compressor check if it has all the necessary bits for your own safety.It will also be easier to sell later on if you need to.

this one has a drain plug also a big hexagonal bung in one end - around 30mm
should depressurise it, screw it out and see what i can see inside?
or should i just scrap it and buy another $200 job?

the water that came out when i drained it was thick with rust

it still pumps up pretty good - never done a lot of work - i've sprayed about 10 guitars over 3 years

now i am worried its gonna blow me up

Sounds like you have a tank which is of some quality, I would think it can be easily fixed to a standard where it will give you lots more years use.

Remove the bung and look inside with a torch, the bottom of the tank should be the worst area. If it was designed properly the tank should sit so the lowest point is where the drain tap is. water will be around it more than anywhere else. Unless you can see heavy rust flaking I would think a good clean out would fix your problem.

Depending on you, you can just tip it up and try and get as much muck out of it as you can or you could give the tank a real good clean out so its as good as new. With all the motor etc on the top it will be heavy but if you are able to remove it the tank would be easy to handle.

If it were mine I would try to put some sort of objects inside if it looks like you can shake them out through the bung hole. Anything small and hard , such as nuts or small blue metal etc add some metho put the bung back tape of the other open holes and give it a really good shake every which way. A friend did this with a very rusty petrol tank using a small concrete mixer which he ties the tank to. he left it running for a while the tank was sparkling clean ( and in his case showed where the rust holes were) Or if its the right shape roll it around the lawn.

Locking inside will also help you determine if the strength has been affected, the thickness should be good enough that the rust would not have weakened it at all unless it really has been rusting badly. A few good taps all over the tank would give you an idea as to how weak or stong it may be as well as loosening the rust flakes if any.

After the shaking open the bung and clean out the muck repeat till its good.

Unfortunately its hard to coat the inside with anything to stop it from rusting further as it isn't possible to see everywhere and know the surface will coat properly, if it hasn't you risk getting paint or whatever coming through with the air in the future.

You could rinse the inside of the tank with a proprietary Phosphoric Acid based metal pre-treatment this will kill and clean the metal so when the acid being tipped out is clean you know its good inside. Unfortunaltely its a tempory fix as water once on the metal will start the rusting again. Your compressor motor should add a small amount of oil into the tank which will slow the rust down but I would not be putting extra into it as oil will mess up the air and can get through to your paint or oil up your filters quicker.

I don't think you need to rush out and buy a new compressor the same problem will still be there, You don't appear to have done much work with yours and it would be expected that it would be working a lot harder and for longer than you appear to have.

As for the danger if it does rust thorugh its more likely to be a tiny area which would spring a leak before a whole area flies out in shrapnel form. You need to have a long line to your gun not short from the compressor as mentioned before so put the compressor away somewhere so its not a danger if you are worried about it.

Good luck spraying!!!

PS are you thinning your lacquer before you strain? it runs through easier ( obviously) but it also dissolves any of the tinters used in the colour which often go hard in the mixture as they settle out in the tin _ especially heavy ones such as reds/ yellows

what about tipping rust converter into the tank and rolling it around?

Just thought about this stuff, maybe it'll work in your application.
If it's good for fuel tanks, should be OK for an airtank

http://www.ftrs.com.au/fueltanks/red_kote.php

rust converter is phosphoric acid based.