Why Finishes Crack & How to Fix ItPaul S
Cracks are one of the many ways that coatings (film forming transparent finishes and paints) can fail. They occur in a variety of forms to include checking, crazing, cracking, alligatoring, and mud cracking. All of these forms are the result of stress.
Checking is a mild form of cracking and doesn’t go all the way through the finish. It often forms irregular, non-linear patterns called ‘crows feet.’ Crazing is similar to checking though the breaks are deeper and wider. As the finish ages, the checking will likely get deeper and go all the way through the finish.
Cracks come in many lengths and form various patterns of intersecting lines that predominately run in the direction of the wood grain, brush strokes, or spray patterns. They go all the way through the coating to the next layer or substrate below. Cracks allow water and water vapor to get in and the moisture causes the wood to swell which then causes the finish to start peeling. The cycle gets progressively worse (like the blue paint in the picture).
Another type of breakage, called cold checking or cracking, is caused by exposing the finished item to freeze/thaw cycles. If the coating was not formulated to handle these extremes, it will crack. Don’t leave your guitar out in the car it the temperature is below freezing and then bring it in to play a song by the wood stove or you may get first hand experience with this type of cracking.
Alligatoring is similar to checking with fissures that are wide at the surface. On rare valuable antiques (emphasis on rare), crazing/alligatoring is considered part of the patina and is generally left untouched. Otherwise it’s considered a nuisance and distraction.
Mud Cracking looks a lot like alligatoring and often occurs immediately following application of a waterborne paint or finish. The wide cracks appear as the coating is drying and happens when it is applied too heavily or the solvents flash off too quickly. The cure for this type of cracking is to sand it back and re-coat as soon as it’s dry enough.
There’s a specialty finish called crackle lacquer that’s designed to produce cracks for the visual effect. It simulates the look of old paint, but a clear coat is used over the crackle layer to protect it and the substrate. The size of the fissures are controlled by how heavily it’s sprayed – heavy coats create larger cracks. Latex paint can also be crackled by applying it over hide glue or a similar base.
What Causes Cracking?
Where do the cracks come from? If the finish isn’t old and brittle, then there’s a good chance it was applied wrong. Some of the most common mistakes that lead to cracking include applying the finish too thick (common with conversion varnish), using a hard/brittle finish over a softer/flexible finish, or using the wrong finish for the job.
All coating failures, including cracking, fall under one of four broad categories, or a combination of them. The causes of cracking include the following;
Defective formulation or mixture
- The coating is too brittle – may be caused by the wrong solvents, plasticizers, catalyst, resin package, pigments, or excessive filler, etc..
- Product is too old.
Improper preparation or application (operator error)
- The coating was applied over an existing cracked surface.
- The solvents in the new coating caused the existing coating to shatter.
- The moisture content (MC) of the substrate was too high (or low) at the time the coating was applied.
- Added too much catalyst or the wrong one.
- Used the wrong solvent(s) for thinning.
- The coating ingredients were not thoroughly mixed.
- The catalyzed material is too old (past its’ pot life).
- The dry film thickness (DFT) is excessive – too many coats, or the coats are too heavy. The thicker the finish, the greater the stress. Conversion varnishes often require a maximum dry film thickness of 5 mils for this reason.
- Not enough flash time was allowed between coats and/or the undercoats were force dried using air movement. When the solvents flash off (evaporate) too quickly, the surface of the finish dries and shrinks faster than the full depth creating a lot of tension which can lead to checking or in extreme cases alligatoring.
- The temperature was too cold during application and/or cure.
- The coating was over-baked (too hot or too long)
Wrong coating for the application
- Too brittle – not flexible enough for the intended purpose. For example, wood expands and contracts – especially outdoors – and the finish has to be flexible enough to stretch with it without breaking.
- Used outdoors but wasn’t formulated for exterior use.
Environmental stresses too extreme
- Excessive moisture/humidity
- Excessive heat or cold
- Excessive UV exposure
- Vibration, impact, flex, substrate expansion/contraction, or other external mechanical stresses
- Ageing. The surface of a coating is exposed to greater direct environmental abuse (air and oxygen, wetting and drying, hot and cold, etc.) than the body which causes the exposed polymers to oxidize and harden faster – this leads to checking. As the coatings becomes more brittle through its depth, it will develop cracks once it can’t stretch enough when the substrate expands and contracts.
Coatings are under constant internal stress and if anything is out of balance, the coating will crack (or delaminate) to relieve the pressure. When a coating is first applied, the liquid solvents start to evaporate and the coating shrinks. If the coating is catalyzed, it also starts to cross-linking during this period as it changes from a liquid to a solid film which increases the amount of shrinking. The coating quickly reaches a point where it can’t flow anymore because the solvents have evaporated and it’s adhered to the substrate (or previous coat of finish/primer/sealer). Until the coating is fully cured, it continues to shrink in thickness, but not in length and width (the two dimensional plane of the substrate). Because it’s stuck to the substrate, the coating pulls but doesn’t move. This constant tension is called internal stress (aka, tensional stress) and is permanent. The thicker the coating, the stronger the stress force will be (which is why coatings applied too thick crack easily).
If the stress is stronger than the cohesive strength of the coating, it will crack to relieve the pressure. On the other hand, if the stress is stronger than the adhesion, the coating will delaminate (peel). If everything is in proper balance, the coating will simply be under permanent internal stress. In some cases, the shrinkage stress of the topcoats can cause the sealer/primer layer to fail by cracking or delaminating if it’s weaker than the topcoat.
If additional external stress forces are applied to the coating and/or substrate (e.g., heat, humidity, expansion & contraction, bending/flexing, impact, etc.) the coating will fail. These external forces are often much larger than the internal forces and cause immediate failure (e.g., impact fractures or flexing cracks). Additionally, age and weathering reduce the cohesive strength and flexibility of coatings which can cause them to crack (which leads to peeling/delamination).
Should the Cracks be Fixed? How?
Faced with an object that has a coating with cracks, you have 3 options;
- Leave it alone
- Repair it
- Remove and replace the coating
Should you fix it? If the item is a rare antique that is more valuable with its original finish, then you should leave it alone and keep it in a place that is climate controlled to minimize the external stresses. If it’s not a valuable antique (most aren’t), then you have the option to leave it the way it is or fix it. Consider that the cracked finish will allow moisture, water and dirt to get into the cracks which will swell and damage the wood which will cause the finish to start peeling. However, if you like the way it looks and there’s no concern about additional damage from water or moisture getting in the cracks, then leave it be. If you decide you want to give it a new look or protect the wood from damage caused by normal use, then go ahead and fix it. Usually you’ll have to remove the old finish and apply a new one.
In cases where the original finish is lacquer or shellac, it’s possible to save the finish through a repair technique called re-amalgamating. Because lacquer and shellac form a film through evaporation of the solvents, and no cross-linking takes place, they can be re-dissolved and flowed out using the proper solvents. Sam Allen’s book, “Classic Finishing Techniques” has a nice section on the process. This process doesn’t always work well and takes a fair amount of practice, so be prepared to completely refinish the piece if you decide to give this technique a try.