Bare metal thickness is the total thickness that the bare metal will be after the removal of any coatings.

It is also known as base metal thickness (BMT) and is the standard measurement adopted by the Australian steel construction market in measuring steel thickness. Base metal thickness is therefore used to measure thickness of structural metal. This approach disregards the protective coatings because they do not add to the strength of the metal material. Total coated thickness (TCT) is the sum of the base material and all paint layers. It includes coats such as primer, the clearcoat, etc, that are on top of each other before any stripping or cleaning takes place. So, when comparing, remember base metal thickness is the measurement of the substrate before application of any coats and the TCT total coated thickness is the thickness of the substrate, including all applied coats.

How thick is bare metal?

Bare metal is normally measured in microns. A micron is 1 millionth of a meter or .00004 inches. The thickness of bare metal can vary greatly depending on the alloy used. Aluminium, for example, is an average of about 4.5-6.0 microns thick, depending on its alloy type and manufacturer.

Steel (galvanised or not) is usually 48 to 52 microns, and stainless steel is 40-50 microns. Metals like chromium and nickel take more coats to paint over than aluminium. They also require a high build primer in order to get enough adhesion between the metal and the top coat.

Chromium-based metals such as bright polished chromate conversion coating range from 80-100 microns thick before any cleaning take place. Chrome plates can actually be 200 microns thick without any plating bath compounds on them. The reason for this thickness difference is that these types of coatings are usually applied in multiple coats. Chromate conversion coatings, for example, are usually applied in 1-3 coats. The more coats present on the metal, the thicker it will be before any cleaning takes place.

In Australia, metal thickness measurement is based on the Australian Standard, which emphasises the bare metal thickness over the TCT, and this will greatly influence the cost of your new shed.

How thick is clearcoat?

Clearcoat is generally about 30 microns thick or .0011 inches when measured over a black substrate (primer). If you have other colours underneath your clearcoat (such as yellow base), then this thickness will increase slightly depending on how many layers of colour there are under the clear. If you want to get an accurate figure for total combined thickness, including paint and primer/paint, it's best to measure each layer separately since they all add up to make one coating system and then add them together.

How thick is primer?

Primer is usually applied in 2 coats of 10-12 microns each and must be sanded twice before any paint or clearcoat can be applied. This means that you have a combined thickness of about 22-24 microns over bare metal. If you add a further 30 microns for the clearcoat, then your total coating system thickness will be 52-54 microns.

What is total coated thickness?

Total coated thickness is simply the sum of all the layers that make up the finished coating system. Coating thickness measurements can be taken from a paint depth gauge or a physical sample of metal/paint by using a micrometer. In this case, you would add together your paint, primer, and clearcoat measurements to get the total combined thickness over bare metal. Typically, there are coating thickness standards that you can refer to when interpreting metal coatings. Before applying additional coatings to building materials, the key things to consider include the coating manufacturer's specs and the environment where the metal will be used. If the substrate is used in harsh conditions, more coats are required.

How is total coated thickness measured?

Total coated thickness is generally measured by using a paint depth gauge (not to be confused with measuring film build, which looks similar) or by using a micrometer. To use a paint depth gauge, simply apply the "inside" tester onto your coating system and push it into the surface until it touches bare metal. Then take note of how many marks are above the panel to get the total combined thickness, including all three layers. The paint depth gauge is only effective if you are sanding to bare metal. You can also use a micrometer on the V of the "inside" tester in order to get accurate thickness readings. Make sure that you measure crosswise with your reading for more accurate results. This is recommended if your sanding through multiple layers to get to bare metal since sandpaper can sometimes take off too much material or leave sanding marks.

What is a micrometer?

A micrometer is a tool used to measure the coating thickness of metal (or other materials) by applying it crosswise with your coating systems. The measurement will be how thick each layer of film build is at that specific point across the surface. This means that you can get an accurate reading of whether or not you are sanding through multiple layers. If there are no layers underneath, then the depth probe will be 0mm across bare metal, just like when using a paint depth gauge. You can use either method for measuring total combined thickness, but one will give you more accurate results than the other since tools such as sandpaper and paint depth gauges alter your coating system slightly. When determining which method to use for your coating system, keep this in mind.

How is primer sanded?

Your primer must be sanded twice, the first time, it will be around 220 grit, and then its final sanding is usually done with a 400 grit paper. This means that the majority of the fine scratches from sanding are removed before any paint or clearcoat can be applied. The more coats of paint, the thicker it will have to be before you can even begin cutting through with your sandpaper. Keep this in mind when deciding how many coats you need.

For accuracy purposes, measure or calculate total coated thickness after priming. Sand twice to eliminate all metal finish marks for better precision on measuring devices such as a paint depth gauge or micrometer. Sheds made of colorbond will not require any preparation.

What is the thickness of a roof sheet?

The thickness of the roof sheet is roughly about 50 microns. If you were to add on 30-40 microns for paint and another 20-30 micron layer of clearcoat, your combined coating system would be measured at anywhere from 80-110 microns over bare metal.

Different measurements of base metal thickness determine the application of the roof sheets. Thicker roof sheets are for industrial applications, whereas thinner ones are ideal for shed roofing.

Consult your local metal materials supplier if you are unsure what a measurement like BMT refers to or how thin or thick the roofing material should be.

How thick is the finish coat?

A standard finish coat (a top coat) usually applies around 90-120 microns but can be as thin as 70 microns depending on how many coats are necessary to achieve desired results. The thicker it gets, the more protection against wear and tear that you will receive.

To achieve a thicker topcoat, you must apply more coats of clear as the thickness gets higher. Keep in mind that the more coats added onto your coating, the more chance for peeling, so keep this in mind before going too thick on coating layers. Check with your local council to ensure if your shed needs planning permission. Tweed Shire Council has a handy guide to follow.