The main determinant of the final result is the chemical nature of the peeling agent itself. Some peeling agents interact with skin biology and are involved in biochemical and pharmacological actions beyond the physicochemical action (specially those derived from natural sources), that is, retinoic acid acts in nuclear receptors (Cucé et al. 2001; Ivanov et al. 2006; Khunger et al. 2004; Nolting et al. 2009), salicylic acid (Dainichi et al. 2008; Imayama et al. 2000; Marczyk et al. 2014), and pyruvic acid (Caperton et al. 2012; Cotellessa et al. 2004; Marczyk et al. 2014) have anti-inflammatory actions. Croton oil is pro-inflammatory and pro-proliferative agent. Phenol , acetone, and ethanol can be used as solvents. On the other hand, some chemicals produce only chemical coagulation and have no other biochemical effects, such as trichloroacetic acid (TCA) (Dainichi 2003), which confers some amount of safety and predictability of results.

Some agents have isomers with different actions, that is, l-lactic acid and d-lactic acid. The isomeric proportion may influence the final result as well.

The strength of the solution is determined by the concentration and so are the expected results. However, in some scenarios “less is more”; this means that there may be one exact concentration to an expected effect. Overconcentration may incur in adverse effects or more pronounced side effects.

All solutions have a saturation point, which is the maximum concentration at a given temperature and vehicle. Some peels are used in an oversaturated solution or “suspension” intended to further interact with the skin, as a “leave in” for some hours.

Stronger effects occur with higher percentage of the agent.

Some solutions exhibit different penetration in the skin at different pH and temperature, so it is important to control both the pH and temperature of the final solution. Some solutions crystalize upon refrigeration.

Stronger effects occur with lower pH and higher temperatures of the solution.

Vehicles may evaporate during the procedure, causing a self-limited exposure to the solution, or may provide penetration of the solution in the skin. Some vehicles provide occlusion effect with extra penetration of the agents. Some vehicles dissolve oil in the skin.

Stronger effects occur with oily and solvent vehicles.

The skin of each patient should be treated as a mildly modifiable ambient. Some pretreatment regimens can affect results and therefore should be prescribed in advance.

The ideal skin for chemical peel s in general is healthy, soft, young, non-oily, phototypes I–II, without irritations on the procedure day. Some of these characteristics can be acquired by directed treatments as a pretreatment regimen. Beware that skin irritation may induce hot spots and adverse effects of peelings. Careful examination is mandatory before every peel. Postpone the procedure if needed.

Balance and health should be goals for skin preparation before chemical peels.

The skin has its own defense mechanisms against natural chemical agents. One of the most important is the lipid layer. It dissolves and attenuates soluble agents and efficiently repels hydrosoluble agents. Therefore, it is important to remove oil from the skin before applying chemical peels. There are many efficient ways. The most efficient way is to wash the face with a detergent and rinse well. After that, rub mildly but firmly a soaked pad with acetone, ethanol, or ether–ethanol solution in the skin, especially in sebaceous areas such as the nose, eyebrows, cheeks, forehead, and chin. The agents mostly affected by the skin oils are TCA and glycolic acid , which are hydrophilic aqueous solutions.

Areas with increased sebum production heal faster and are thicker. Those are the areas that need extra penetration of the chemical agent for uniform results.

Try not to irritate the skin during degreasing to prevent hot spots, especially on superficial peels. Beware of seborrheic dermatitis or any other dermatitis before peeling with superficial agents. Skin inflammation causes hot spots and may hurt when using the degreasing agent, which may focally increase skin irritation.

Always degrease in the same way for every peeling agent. So you can vary penetration during the application of the agent.

Stronger effects occur in non-oily skin.

Some chemical peels such as phenol peels are not recommended for high phototypes (>IV) due to the high possibility of persistent dichromic macules and scar formation. Luckily, in real life, these patients do not present with complaints that would require deep chemical peeling. Post-inflammatory hyperpigmentation (PIH) occurs more frequently on intermediate phototypes (III/IV).

The use of hydroquinone for at least 1 month reduces incidence of PIH and should always be prescribed for phototypes III–V. The use of potent topical steroids on the night before and for the following 2 weeks may prevent PIH.

The patient should stop any irritant prescription at least 48 h before peeling. Benzoyl oxide: at least 1 week before. The use of potent topical steroids after peeling may trigger severe acne , rosacea, hair growth, and telangiectasia (Hengge et al. 2006). Therefore, the risk–benefit should be accessed on individual basis.

The application technique is the variable that can be mostly changed among all the factors to a given expected result. A single, very fast, very light, semidry stroke of TCA 90% may confer results similar a slow, high pressure, wet application of TCA 10%.

The size, thickness, and smoothness of the applicator confer additional characteristics; in the same way, a painter may use a roller, spray, or a paintbrush. Some paintings may require multiple passes, and some may require only one thin pass. It is all part of the art.

Stronger effects occur with more volume, more friction, and more exposure time.

All the applicator s used in chemical peeling should be disposable (Fig. 2).

Cotton pads or cotton balls generally use soft applicators. These cannot be used with acids that deteriorate the protective gloves (TCA over 50% or phenol). If they are ripped into smaller pieces, they present the advantage of saving important volume of the solution.

Goat hair disposable brushes are excellent for peels with fast and soaked applications such as Jessner’s solution and retinoic acid. When using retinoic acid-tinted solutions, the color becomes more uniform as cotton absorbs and filters the solution and the suspension particles.

Cotton-tipped plastic applicators (Q-Tips) present an internal deposit of liquid inside the plastic hollow handle. The liquid accumulates by capillarity and lasts longer without drying the cotton. Q-Tips are great to absorb tears during blepharopeelings.

Cotton-tipped plastic applicators are very easy to be oversaturated and drip during application. These are not recommended for blepharopeeling.

Rayon large applicators are great for both glycolic and TCA peels. The rayon serves as a sponge, and the plastic handle is larger than a usual Q-Tip, which confers less capillary effect. The liquid is delivered in a uniform way.

Cotton-tipped wooden applicators are less susceptible to dripping than the plastic ones. Are great for blepharopeelings.