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The following in-depth guide explains how to sand signs and panels without the use of power tools. Ensuring your oil and water based paint coatings adhere and dry smoothly. Protecting plywood, hardwood, MDF, aluminium, steel, and plastic substrates.
Why sanding signs is important
Imagine you can see your sign or panel surface magnified by 1000%. On porous substrates like untreated wood, there’ll be lots of tiny crevices. On non-porous substrates like steel and acrylic, there’ll be fewer tiny crevices or none at all.
Sanding scratches the surface which provides a ‘key’. Keying enables the coating to attach itself to the scratched surface below.
Now imagine layers of coatings on your surface. Seeping into each tiny pore and crevice. The coating(s) gradually cure and solidify into one protective skin.
Sanding before and between coats protects your substrate. As well as leaving an overall smoother finish. Especially important for signs and display panels.
Fail to prepare, prepare to fail
It’s wise to prepare your surface and protect your health before sanding.
Work outside or in a well ventilated space with enough room to manoeuvre around your surface. Sanding can be messy. Wear a protective suit or clothing which you don’t mind getting dirty.
Wear a dust mask if you’re sanding:
- In a poorly ventilated space
- Over prolonged periods of time
- Dry – without the use of water
- Surface has been treated and flaking or chipping away
Check your surface is clean and shows no traces of dust, grime, oils, or residues. Surface contaminants clog your sandpaper and can repel your coating. Wash the surface with warm soapy water if it’s dirty. Do not soak bare (untreated) woodgrain, as water can damage the wood.
On previously treated surfaces, use isopropyl alcohol, glue remover, or citrus based cleaners to remove stubborn grime.
I don’t wear gloves when sanding as I find them to be a hindrance. However, protect your skin when handling solvents.
The sanding surface
Make sure your sign or panel is stable and at a height, you’re comfortable working on. If the sign or panel wobbles or bends, secure it onto a rigid surface beneath. Create a worktop with a sheet of plywood or timber batons. Alternatively, use clamps to secure the surface to your working height.
I use wooden trestles at waist height to place panels flat. At waist height, I can reach over most panels to sand a larger area. If I want the surface higher, I attach stilts made from wooden batons.
Sanding untreated wooden and MDF surfaces
Sanding is not necessary before painting bare, smooth wood. The initial coatings will seep into the surface and most often raise fibres from the woodgrain.
However, sand the wood if the untreated surface feels coarse. And watch out for splinters. Sanding the bare coarseness will minimise removal of the first coating, once sanding between coats.
Sand the surface in the direction of the grain when using wood stain, clear varnish, or other semi-opaque coatings. The scratches made from the sandpaper are liable to shine through your first coat.
Always sand a test sample whenever possible. The result will allow you to decide if your sandpaper is too coarse or your sanding pressure is too hard.
Sanding untreated plastic and alloy surfaces
It’s sensible to key the surface of non-porous metals and plastics before your first coat. Read the paint manufacturer’s directions and ask for advice from your paint merchant.
Your first coat has to bond onto the surface before any following layers can be confidently applied. Sanding and removing the dust will help to achieve a stronger bond.
Sanding treated surfaces
When repainting or refurbishing signs and panels, sanding previously treated surfaces is vital.
A light sanding is adequate when the coating on the sign or panel is in good condition. Coupled with the appropriate paint for recoating, keying the sound surface will give you a clean finish and peace of mind.
Faded, chipping, flaking, and cracking surfaces require extra attention. Filling may also be necessary depending on the condition of the surface. Use appropriate wood or autobody fillers and expect to sand the filled areas too.
Apply filler gradually in deeper areas. Use small amounts of filler and build layers. Excess filler will inevitably have to be sanded off.
Your objective is to smoothen the previously treated surface as best as practically possible. Providing a new keyed surface for your coating to adhere to.
On exterior signs exposed to weathering, moisture can adversely affect the sign’s structure. Seek advice from a woodworker if the sign or its components are obviously damaged. Moss and mould is a warning and may convince you to replace the sign with a new one.
Avoid sanding or filling any wood which feels spongy or brittle.
Choosing sandpaper coarseness
There are many grades of sandpaper coarseness (grit), with the lower number denoting coarser grit.
Sandpapers are commonly graded as coarse (40 to 60 grit), Medium (80 to 120), Fine (150 to 180), Very Fine (220 to 240), Extra Fine (280 to 320), and Super Fine (360 and above).
Smoothening rough surfaces requires rough sandpaper. If your sandpaper isn’t coarse enough, the rough surface will clog the paper’s grit. If the sandpaper is too coarse, it’ll shred smoother surfaces.
180 grit sandpaper is widely available and a good all-rounder. It’s suitable for bare wood, MDF, hard plastics, aluminium, and steel.
Choose smoother sandpaper if you’re coating with a semi-opaque finish or if the bare surface is already smooth. Use finer sandpapers between coats to gradually remove the scratched marks. When painting with opaque coatings, the scratched marks will be less visible with additional coats.
Harsh sanding will show through your coating. Always test new products to gauge their efficacy.
How to use sandpaper
Sandpaper is widely available in sheets approximately A4 in size.
Fold the sandpaper in half to create a spine. Make sure the spine is rigid and tear down the crease. Repeat the folding and tearing again with the two halves. You’ll now be left with one quarter piece of sandpaper, approximately 100mm by 100mm (4” by 4”). Fold this in half.
The double sided grit fits comfortably in hand. The grit creates friction and won’t slip easily out of your fingers. When you don’t fold the sandpaper and handle a single (unfolded) piece, it’ll likely lock onto the surface rather than move with your hand.
Gloves can help cause friction and grip the sandpaper better. They’ll also protect your hands from splinters or other sharp debris.
How to use a sanding block
A simple sanding block can be made from a short length of hardwood around 50mm (2”) square. Wrap the sandpaper around the block and grip it onto the edges of the sandpaper. The flat, gritty side will evenly sand your surface.
Alternatively, use a rubber sanding block. The one shown in the photographs has teeth underneath the side wings. The teeth pierce and hold strips of sandpaper whilst the wings clamp shut.
The dense rubber is kinder on the surface and the sanding hand, as opposed to a hardwood block.
Sanding blocks work well on flat surfaces. However, avoid using sanding blocks on angled and radius edges. The flat underside will sand the edges out of their shape.
How to sand angled and routered edges
To avoid misshaping an angled, rounded, or irregular shaped edge, try a sanding pad.
3M Scotchbrite pads are similar to scouring sponges. The maroon pads are suitable for general purposes. They’re flexible pieces of coarse fabric and available in different grits. The fabric moulds into the shape of the edge, allowing the grit to smoothen the edge.
Contour shaped sanding grips are also handy tools for sanding routered edges. Available for different sized grooves and sandblasted areas, sandpaper moulds into their shape. Hold the sandpaper around the contour grips similar to holding sanding blocks.
Sawn timber can be coarse and full of loose splinters. Before using a pad or contour grip, use coarse sandpaper to remove loose debris. Take extra care when smoothening the sawn edge.
Always sand parallel to the woodgrain’s direction when sanding before staining or varnishing. This motion minimises visible scratches which can appear through the stain or varnish.
Select the correct grit and let the sandpaper do the smoothening. Coarser surfaces will require increased hand pressure.
Take care on softer surfaces such as softwood and foam PVC. Too much pressure with high grit sandpaper can cause shallow dips on the surface. These dips will be magnified after coating, especially with high gloss finishes.
If you’ve sanded the surface too hard, skim the surface with an appropriate filler. Sand the cured filler gently until the surface is smoothened and even.
Using water with sandpaper has four immediate benefits:
- The sandpaper’s use is prolonged.
- Wet sandpaper doesn’t clog quickly with dust particles.
- Sanded areas are dust free and the wet dust is easily cleaned.
- It can be argued that a finer sanded finish is achieved with the use of wet sanding.
Avoid causing puddles on your sanded surface. Sand manageable sized areas. Dry each area as you sand and observe the drying process. You’ll notice un-sanded areas once the surface is fully dry.
I avoid using soap or detergent when wet sanding. I don’t deem these necessary for the surfaces which I’m working on. I’m also cautious to introduce potential contaminants to surfaces which I intend to paint.
In the past, I’ve used soap and water to sand metal panels then washed and dried thoroughly. But I would not use wet soap on untreated wood for fear of the soap seeping into the wood’s core.
If you plan to stain or varnish bare wood, keep in mind, some woods can discolour when wet. Bare MDF and wood are also liable to warp when wet, so err on the side of caution.
Always coat bare wood and MDF before wet sanding. Sand lightly and build the layers of coatings. The coated barriers will prevent water from seeping through and potentially damaging the untreated surface.
Previously treated surfaces should appear evenly dull and scratched once sanded. Feel the surface with your fingertips to check for any coarse areas. Repeat the wet sanding process to smoothen again.
Always wash (with clean water) and dry the wet sanded areas. Leaving the sign or panel to air dry before a coating is helpful too.
Removing dust and preparing the surface
Dust gets everywhere. Any remaining surface dust will be collected from your paintbrush and smeared all over your smoothly sanded surface.
Wherever safe and possible, use compressed air to remove the surface dust.
Invest in a handheld, soft (preferably long) bristled brush. A decorator pasting brush will do.
Starting from the centre of the sanded surface, sweep the dust away to the edges.
Remember to remove dust gathered in the corners of frames and mouldings. Don’t forget to dust the sign or panel’s edges and backs too.
Once all visible dust is removed, use a tack rag to mop up the invisible particles. A tack rag is a piece of sticky mutton cloth, often in packets in quantity boxes. Tack rags are available from auto body supply stores, paint merchants, and some DIY stores. The tack rag can be used more than once if sealed and stored after use.
Wipe the tack rag over the sanded area and it’ll collect the remaining fine particles. When the tack rag loses its stickiness, replace it with a new one. Avoid using soiled tack rags, as you may wipe dirt and grime over the sanded surface.
You can also use a spray bottle filled with isopropyl alcohol or clean water. Fine mist the sanded area and dry with a clean, lint-free cloth or absorbent paper. Or dampen a clean cloth and wipe the sanded surface, collecting as much dust as possible.
Start as you mean to go on
Understanding the balance between sanding pressure and coats of paint takes time. When working with paints for sign making, you’ll soon build experience sanding.
Get to know the opacity of the paints you are using. Some paints will not cover scratched areas and will require coating more than once. Unless there are imperfections in your coat, go easy when sanding. Or you’ll get caught in a cycle of sanding and scratching the surface, before coating over again.
On occasion, some coats are not worthwhile sanding as the paint coverage is too thin. When using poor covering paints, reduce the sandpaper grit.
Work safely and as clean as possible at all times.
Reduce dust intake wherever possible.
Sand gently and watch out for splinters or loose debris.
First, test the coarseness of sandpaper on a sample.
Allow the coating to harden and check if you need to sand with more pressure.
Remember to build layers of paint that will key into each other.
Remove as much dust as possible.
Thanks for reading my guide on how to sand signs and panels. Get in touch if you have any questions.