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White rock salt

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BOB BLACKSHAW01/10/2020 11:02:45
402 forum posts
87 photos

Hello, as its getting near the damp season for the shed I was wondering if white rock salt in a large tray would help with damp. This salt can be brought in 25kg bags at a cheap price, but pure rock salt is quiet expensive. My shed over the years hasn't been to bad as its foil insulated with a blow heater for air circulation when the temperature drops, just added help for dreaded damp . I wipe all exposed surfaces with WD 40 but always a slight rust mark has appeared over winter.


SillyOldDuffer01/10/2020 11:28:53
7221 forum posts
1591 photos

I wouldn't - salt is corrosive.

A few things to try:

  • How does water get in? Might be because the roof or walls are porous, or a gutters and a drain are needed. Failed or absent damp-proof course? Leaks? Eliminate the source including fuel burning heaters. Ideally don't breath!
  • Stop the temperature changing. Condensation occurs when warm wet air hits a cold surface. Much less condensation when cold wet air hits equally cold metal. Bad heating and cooling occurs naturally due to daytime warming and night time cooling, and also due to random artificial heating. Either keep the temperature constant, or keep the inside temperature the same as outside, or at least only change it slowly. Add the thickest and best insulation that can be managed,
  • Remove water with a dehumidifier.
  • Improve ventilation so damp air can escape. One trick is to leave a gap at the bottom of glass windows so condensation drips outside.


bernard towers01/10/2020 11:34:04
188 forum posts
71 photos

Wd40 is not the thing to use for rust protection as it has kerosene in ,far better to use a metal protection fluid from rocol or similar suppliers. Or you could just use hydraulic oil which is impervious to water. My unheated and uninsulated garage has three machines in it which use hydraulic oil as a cutting oil and I never see any rust. I get my hyd. oil from the local tractor dealer after they have done a hyd. oil change.

david homer01/10/2020 11:40:48
30 forum posts

Hi, I have used the moisture absorption trays/traps and crystals available from Wilkos, Robert Dyas etc and they work very well in my workshop which is electrically heated so no moisture from that. The moisture in a well sealed shop is from your breathe blow lamps and carrying in water on a wet day surprising how much moisture they collect. We also used to put them in a narrowboat we had over the winter but they did not collect much which I think was due to the boat being colder.


BOB BLACKSHAW01/10/2020 11:58:02
402 forum posts
87 photos

Thanks for the replies, I have no problem with leaks or anything like that, its temperature change that could be the problem. I read on the web that if you get two buckets drill holes in one fill with salt and put that in the other bucket water will be absorbed from the atmosphere. That's why I thought of the 25kg bag, but if its not going to work hydraulic fluid looks a good option, surprised about WD 40, I will remove that today. I cover my machines at night for air protection, I have a 1967 MGB in the garage I have a Carcoon but for the last ten years I haven't used it as I cover the car over and put a fan underneath and have had no damp problems, also have a double carpet on the floor. Keeping a constant air temperature seems the answer for me in the shed


Martin Kyte01/10/2020 12:10:41
2406 forum posts
40 photos

I would argue that the foil is a bad idea. In winter the amount of moisture in the cold outside air is low. I would suggest that assuming there are no gross water leaks when it's raining the source of moisture in the workshop is you and the foil is adding to the problem by trapping moisture inside. Unless you heat your workshop on a continuous basis the best thing you can do is fit an extractor fan on a timer. Turn it on when you leave and let it run for 30min to an hour to exhaust all the warm moist air and replace with cold dry air.

Having plenty of unpainted wooden surfaces also helps to buffer humidity changes.

regards Martin

duncan webster01/10/2020 12:43:09
3246 forum posts
56 photos

Salt is only corrosive if it is dissolved in water and in contact with the metal. They use salt mines as repositories because it is dry and constant temperature.

When I had a rust problem I used Duck Oil. No idea what's in it, but it came in 5l tins so was fairly cheap, and certainly worked even if it was a bit messy.

Best bet is to control humidity, insulate and keep above dew point as per article by SOD in ME some time ago. Martin's suggestion is worth a try, but what about when you first go into workshop, the machines are cold and you are breathing damp air over them.

JA01/10/2020 13:03:27
1153 forum posts
72 photos

The MEW of November 2003 gave details of a very nice heater for a lathe, milling machine etc. It ran off the mains and heated the machine by three, in series, 1 ohm 50 watt resistors bolted to the structure. These were fed with 10 to 14 volts and were turned on by a adjustable temperature sensor.

There is nothing clever about it and the parts are cheap. I rather like it but have no real need for it.

Perhaps it is time for a reprint in MEW?


not done it yet01/10/2020 13:07:24
5944 forum posts
20 photos


Edited By not done it yet on 01/10/2020 13:08:24

Circlip01/10/2020 13:21:33
1269 forum posts

Can you still get piano heaters? Old standby was to cover the lathe etc. and stick a low wattage light bulb under the cover. Only woks with Tungsten filament bulbs, NOT Halogen or LEDs.

Regards Ian.

Frances IoM01/10/2020 14:22:23
1118 forum posts
27 photos
some time ago I acquired 3 electrical vivarium heaters - each being a dual flat mains-fed resistive panel of about 30W heat dissipation and 18" square - my thought was to attach one to rear of splash panel of my lathe - as it turned out my cellar workshop on chalk ground keeps dry I haven't needed extra heat - I don't know how much the panels cost normally but they were sold for about 50p each at the auction.
BOB BLACKSHAW01/10/2020 17:56:08
402 forum posts
87 photos

I have a spare new bathroom fan, this suggested by Martin looks a good idea, thanks.


mark costello 101/10/2020 21:04:57
646 forum posts
12 photos

How does One get a Duck to stay still to be milked? wink

Bazyle01/10/2020 22:08:51
5896 forum posts
218 photos

An extractor fan is a bad idea. It implies another hole to let in moist air when a warm front passes by.

V8Eng01/10/2020 22:12:56
1597 forum posts
32 photos
Posted by mark costello 1 on 01/10/2020 21:04:57:

How does One get a Duck to stay still to be milked? wink

Preferably using a method that keeps them suitable for cooking with orange sauce🤐

Emgee01/10/2020 22:21:57
2045 forum posts
258 photos


The double carpet on the floor could absorb moisture and be the source of dampness in your shed, worth checking.


Grindstone Cowboy01/10/2020 22:28:06
620 forum posts
57 photos
Posted by V8Eng on 01/10/2020 22:12:56:
Posted by mark costello 1 on 01/10/2020 21:04:57:

How does One get a Duck to stay still to be milked? wink

Preferably using a method that keeps them suitable for cooking with orange sauce🤐

By using one of these, of course (only joking, but it's not far off)


Steviegtr01/10/2020 22:54:50
2073 forum posts
287 photos

On a boat i used to have it came with 2 square boxes around 6" with a small hole in the top. These were half full with salt.

When they were full absorbed it was a simple matter of taking the top off & drying out to use again.


BOB BLACKSHAW02/10/2020 09:11:47
402 forum posts
87 photos

Good Morning all, again thanks for the input on this, we have seemed to come in a circle as Steve used salt on his boat.

I know that temperature difference from cold to warm will produce damp if the rise is to quick for the cold surface to catch up with the rise in temperature.

Example is a concrete garage floor, if it's been very cold over a few days then it turns warm, which our climate does the garage floor is wet. I found it quite funny many years ago when my neighbour had a carpet in his garage, but I have two carpets in mine now as it's a insulation for the slow rise in temperature needed for my MGB.

As stated in my original thread my shed is not to bad , two layers of floor insulation. a blow air heater set a low temperature, and is lined with a foil bubble material, but I've read that salt in a container will take out any moisture in the confined space.

I would like to give it a try, but is a 25kg of white rock salt pure salt ?

Thanks Bob

SillyOldDuffer02/10/2020 10:06:20
7221 forum posts
1591 photos

If you want to try a bag of chemicals, Calcium Chloride is the stuff. It absorbs enough water from air to dissolve itself.

Sea Salt and Rock Salt are mostly Sodium Chloride which doesn't absorb water. But being a natural product from the sea, Sea & Rock Salt contain small quantities of other chemicals that do. Not much, but enough to cause the crystals to clump damply together.

As clumping is a nuisance in the kitchen, table salt is refined to remove water absorbing chemicals. Useless as a desiccant. For the same reason Dishwasher Salt is no good.

Beware of the word 'salt'. To the ordinary man it means Sodium Chloride, but technically any ionic compound is a salt. Caustic Soda, Saltpetre, Potassium Cyanide, and Chlorate Weedkiller are all Salts. Don't sprinkle them on your dinner! I'm not sure which salt Stevie used on his boat, or what the purpose was.

'Alcohol' is the same, and misunderstandings once led to many poisonings. Ethyl Alcohol is a moderately safe recreational drug; Methyl Alcohol is poisonous. The modern names are safer; Methanol, Ethanol, and Propanol, are obviously different, and it's not necessary to be a Chemist to understand Ethylene Glycol (Anti-Freeze), shouldn't be added to wine.

Silica Gel is more popular as a desiccant than Calcium Chloride because it doesn't liquefy, and is easier to manage.

I'd rather invest in an electric dehumidifier!


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