St.Albans and District Model Engineering Society

Workshop & Rust

A common question on forums concerns rust prevention. This flows on to heating, insulation, ventilation and dehumidifiers.

Quick answer

The best answer is to remove the moisture and stop it getting in again. That means get a dehumidifier and totally draught proof your shed.


Someone always comes along who claims to allow a howling wind through the shed in all weathers and never gets any rust. This is simply not true. What he must be doing is thoroughly oiling the shiny bits he can see and is not aware of the rust forming on the parts out of sight.
The physics are quite simple. When it is cold outside the machines and everything will cool down. They at this point will not be damp because the cold air is low humidity and it will look as though the practice of allowing the air to circulate is working. However when a warm weather front comes in it will be damp, that’s why it is warm, in fact it will probably be raining and the air will be at 100% humidity. When it hits the machines it will condense water out while slightly warming them up a bit. On the oily parts it will not be a problem and will just sit there, likewise painted surfaces, but the unseen corners will be wet, not just damp but quite wet. So the problem manifests not during the cold spell but when it gets warm again. After a while the weather eases off and the air is dryer so a draught will dry the machines off again so the period of dampness may not be apparent but it has happened and had an effect. You can see this happening in lots of covered rain free places that are open to air such as underpasses, public toilets, garage floors etc.
It can seem worse if the shed is not deliberately open but not sealed either because the process slows down and the moisture stays around longer and is visible when the owner goes out to the shed.
Heating can help. Particularly as it encourages insulation and a by-product of that is improved sealing. The heat means that the air coming in will be enabled to carry more moisture, including that sweated and breathed out by the user, up to a point.. However fixed output heating will allow the whole shed to go up and down in temperature as the outside temperature fluctuates. This may mean as the air cools it does drop below the dew point and cause condensation, particularly when a cold snap follows a moist damp phase and the owner using the shed.

Draught proofing

Even bricks are air porous but the main problem is wall-roof interface and doors. Nothing beats a sheet of plastic over every surface with taped joins all the way round but few buildings allow that to be done simply. Canned expanding foam is the answer to many awkward corners and joints. Once sealed the temperature can still change but not due to warm moist air entering. If a point is made of allowing an air change during a cold dry day it can flush out residual moisture. However the occupant is still introducing moisture through breathing, sweating, and having steaming cups of coffee. Even heated there will be a limit to the moisture holding content and it needs to be considered. Ventilation is also needed when occupied both to breathe and to waft away oil and paint fumes. Therefore a ventilation hole is necessary, but should be made to be sealed well and will allow the window to be non-opening and so fully sealed. One solution is a 4in drainpipe as positive sealing caps are available but you do have to remember to fit them.


Generally an oil filled radiator is best for continuous use and radiates some of the heat at you and the machines. A convector is about the worst option as the heat first goes up to the ceiling and then works back down to everything else. A fan heater is best for intermittent heating as it can direct heat at you or an object as required. Radiant heaters present a fire risk. Propane heaters are very bad as they produce vast quantities of moisture. An anglepoise bench lamp will probably now be fitted with a low energy bulb but in winter it can be exchanged for an old fashioned one and a small but perceptible heat will be felt by hands working on some small parts. An alternative for continuous heating is to specifically heat the machines using small greenhouse tube heaters of 30-60W. This ensures that they are always one of the warmer things n the shed and the condensation lands elsewhere. The disadvantage is that tools and stock metal is not protected.


Nowadays 2 inches of insulation just isn’t enough even if it is high performance if you are continuously heating. A standard 11 in cavity brick wall with a 4in air gap is equivalent to just 1 inch of polystyrene so 4-6 inches of insulation should be the target. Expanded polystyrene or Kingspan or Celotex is 25% better than fibreglass or rockwool, but rather more expensive yet giving some other rigidity advantages.


A dehumidifier will positively remove moisture. It does not have to run continuously and can be on a timer or better be the type fitted with a humidistat which can be set to 50%. If the shed is sealed it will hardly run at all once the days input of new air and perspiration is removed.Do not get a 'chemical dehumidifier' as they are very expensive to replace the 'salts' and are only suitable for small volumes like a tool cupboard.
It also recovers heat from the moisture 'free'. They run at about 250W while cooling but on average over a day will be 1/4 or less of that. Plus you are probably getting twice the heat value for your money. Next people complain that they don't work well when cold. This s true but they will remove the mositure as the shed is still warm from you being in it after which there is less to keep up with. There is a variety that do not have a compressor. Avoid them as they use a lot of power because they have a built in heater.

Removing rust

If you already have some rust developed how do you get rid of it?
Whatever method you use it is essential to degrease the part first with solvent (white spirit) or detergent.
There is only one sensible economic solution which is phosphoric acid. Let's come back to this later.

Starting with some things not to bother with. Cola and similar fizzy drinks are said to be cheap and will dissolve rust. Actually the active ingredient is the small amount of phosphoric acid it contains so it just makes it a very expensive way to buy the acid.

Next is vinegar. This is a dilute solution of acetic acid and is easily available, and benign to hands and utensils. Although not expensive it is still not the best bargain way to get acid as it is made to food standards so that ads cost. Because it is dilute it is relatively slow acting but that makes it possible to keep an eye on progress and perhaps leave overnight. When used it is helpful to brush over the part frequently to move debris and let the acid get to the part. Still it is not really ecenomical except for one source. ****good idea*** If you come to the end of a jar of pickled onions etc the remaining liquid will contain some vinegar. It is effective and kitchen friendly.

Molasses. There is no obvious reason why this would work and I rather doubt it does but have not tried it. It is possible that some chemicals added for preservation are the active ingredient

Citric acid. This is available from various sources sometimes expensive being food grade but homebrew winemaking and catering suppliers are cheaper. Very similar in use to vinegar. Its big advantage may be that it comes as a solid, crystals, which are dissoled in wateras required. This makes it convenient to store until needed.

Naval Jelly. This is phosphoric acid in a paste so stays where it is put but only the part in contact with the work does anything so the thicker layer is just waste. The 'Naval' part indicates that no consideration was given to efficiency or cost so it is not worth considering if you are the one who pays the bills

Chelates or chelating agents. Sold under one particular brand name particularly in the USA these are slower and work by ion exchange rather than acid disolving the metal. Expensive but may be worth it for controlled processing of expensive and delicate items

Brick and patio cleaner, pH balancer for swiming pools, all are HCL or hydrochloric acid. Cheap and quite common but a bit vicious, even when diluted further. It dissolves the metal and in so doing detaches the rust particles but you are losing metal. Can leave the item looking a pit porous where the rust has gone. It must be neutralised by washing well and preferably washing in mild alkaline solution (washing soda or garden lime solution). The surface is then so clean it re-rusts quickly so immediate post treatment with oil, wax, paint etc is needed. I often give a light phosphoric acid treatment after the part has 'flash rusted'. Important: do not store in the workshop or garage as it gives off chlorine which encourages rust on everything nearby.

Electrolysis Effective and controllable this is one of the better techniques for smaller parts but does not form the protective layer of phosphoric acid

Phosphoric acid This is available by the gallon and smaller quantities and although initially expensive is the most cost effective. Some people want to dunk the part in it but that is likely to be wasteful. It can be painted on and the effect monitored, then washed off. It is a weak acid so does not attack the metal fast but converts the rust into iron phosphate which can form a bit of a protective layer. Depending on elements in the iron, surface contamination and moisture it can go gooey, or produce white or black deposits but that is not a problem. Thick rust tends to form a black patch and needs physical removal.
Speed of operation depends on the concentration etc but is typically less than half and hour, not hours. After washing it is best to dry quickly with compressed air, sun, heat gun to avoid flash rusting. However after longer treatment the layer of grey phosphate on the surface provides a rust barrier. Clean new metal does ot react much so does not form the grey layer.