Weak fingernails are defined as nails with a faulty structure. Brittle nails can be caused by environmental factors, by a lack of nutrition or from an underlying medical condition. Psoriasis, Raynaud's disease and certain lung conditions can contribute to brittle nails. You can fix weak nails in just a few steps.InstructionsTake a biotin supplement each day. Biotin deficiency results in weak nails.Massage a nail strengthening oil or cream into the nails and cuticles. Use a solution that contains protein. Remove any excess oil from the nail and cuticles with a damp towel. Massaging the area stimulates blood circulation in the nails and encourages growth. This should be done a few times each week.Apply a clear nail polish designed to strengthen nails. Use a product advertised as preventing splitting, nail cracking and chipping.Remove nail polish (colored or clear) with a remover that does not contain acetone. Acetone dries out nails and makes them more brittle.Keep hangnails clipped to prevent brittle nails. Also refrain from biting or picking at nails. Biting and picking at nails causes tears, breaks and prevents nails from growing strong.Wear rubber gloves when cleaning. Harsh chemicals dry out the nails and make nails more brittle and more likely to break. The chemicals are also detrimental to cuticles.Source: eHOW.com